This is a long post. Notes on viewing.
About this post
A mathematical object, or a type of math object, is represented in practice in a great variety of ways, including some that mathematicians rarely think of as "representations".
In this post you will find examples and comments about many different types of representations as well as references to the literature. I am not aware that anyone has considered all these different ideas of representation in one place before. Reading through this post should raise your consciousness about what is going on when you do math.
This is also an experiment in exposition. The examples are discussed in a style similar to the way a Mathematica command is discussed in the Documentation Center, using mostly nonhierarchical bulleted lists. I find it easy to discover what I want to know when it is written in that way. (What is hard is discovering the name of a command that will do what I want.)
Types of representations
Using language
 Language can be used to define a type of object.
 A definition is intended to be precise enough to determine all the properties that objects of that type all have. (Pay attention to the two uses of the word "all" in that sentence; they are both significant, in very different ways.)
 Language can be used to describe an object, exhibiting properties without determining all properties.
 It can also provide metaphors, making use of one of the basic tools of our brain to understand the world.
 The language used is most commonly mathematical English, a special dialect of English.
 The symbolic language of mathematics (distinct from mathematical English) is used widely in calculations. Phrases from the symbolic language are often embedded in a statement in math English. The symbolic language includes among others algebraic notation and logical notation.
 The language may also be a formal language, a language that is mathematically defined and is thus itself a mathematical object. Logic texts generally present the first order predicate calculus as a formal language.
 Neither mathematical English nor the symbolic language is a formal language. Both allow irregularities and ambiguities.
Mathematical objects
The representation itself may be a mathematical object, such as:
 A linear representation of a group. Not only are the groups mathematical objects, so is the representation.
 An embedding of a manifold into Euclidean space. A definition given in a formal language of the first order predicate calculus of the property of commutativity of binary operations. (Thus a property can be represented as a math object.)
Visual representations
A math object can be represented visually using a physical object such as a picture, graph (in several senses), or diagram.
 The visual processing of our brain is our major source of knowledge of the world and takes about a fifth of the brain's processing power. We can learn many things using our vision that would take much longer to learn using verbal descriptions. (Proofs are a different matter.)
 When you look at a graph (for example) your brain creates a mental representation of the graph (see below).
Mental representations
If you are a mathematician, a math object such as "$42$", "the real numbers" or "continuity" has a mental representation in your brain.
 In the math ed literature, such a representation is called "mental image", "concept image", "procept", or "schema". (The word "image" in these names is not thought of as necessarily visual.)
 The procept or schema describe all the things that come to mind when you think about a particular math object: The definition, important theorems, visual images, important examples, and various metaphors that help you understand it.
 The visual images occuring in a mental schema for an object may themselves be mental representations of physical objects. The examples and theorems may be mental representations of ideas you learned from language or pictures, and so on. The relationships between different kinds of representations get quite convoluted.
Metaphors
Conceptual metaphors are a particular kind of mental representation of an object which involve mentally associating some aspects of the objects with some aspects of something else — a physical object, an image, an action or another abstract object.
 A conceptual metaphor may give you new insight into the object.
 It may also mislead you because you think of properties of the other object that the math object doesn't have.
 A graph of a function is a conceptual metaphor.
 When you say that a point on a graph "rises as it goes from left to right" your metaphor is an action.
 When you say that the cosets of a normal subgroup of a group "get along" with the group multiplication, your metaphor identifies a property they have with an aspect of human behavior.
Properties of representations
A representation of a math object may or may not
 determine it completely
 exhibit some of its properties
 suggest easy proofs of some theorems
 provide a useful way of thinking about it
 mislead you about the object's properties
 mislead you about what is significant about the object
Examples of representations
This list shows many of the possibilities of representation. In each case I discuss the example in terms of the two bulleted lists above. Some of the examples are reused from my previous publications.
Functions
Example (F1) "Let $f(x)$ be the function defined by $f(x)=x^3x$."
 This is an expression in mathematical English that a fluent reader of mathematical English will recognize gives a definition of a specific function.
 (F1) is therefore a representation of that function.
 The word "representation" is not usually used in this way in math. My intention is that it should be recognized as the same kind of object as many other representations.
 The expression contains the formula $x^3x$. This is an encapsulated computation in the symbolic language of math. It allows someone who knows basic algebra and calculus to perform calculations that find the roots, extrema and inflection points of the function $f$.
 The word "let" suggests to the fluent reader of mathematical English that (F1) is a definition which is probably going to hold for the next chunk of text, but probably not for the whole article or book.
 Statements in mathematical English are generally subject to conventions. In a calculus text (F1) would automatically mean that the function had the real numbers as domain and codomain.
 The last two remarks show that a beginner has to learn to read mathematical English.
 Another convention is discussed in the following diatribe.
Diatribe
You would expect $f(x)$ by itself to mean the value of $f$ at $x$, but in (F1) the $x$ has the property of a bound variable. In mathematical English, "let" binds variables. However, after the definition, in the text the "$x$" in the expression "$f(x)$" will be free, but the $f$ will be bound to the specific meaning. It is reasonable to say that the term "$f(x)$" represents the expression "$x^3x$" and that $f$ is the (temporary) name of the function. Nevertheless, it is very common to say "the function $f(x)$" to mean $f$.
A fluent reader of mathematical English knows all this, but probably no one has ever said it explicitly to them. Mathematical English and the symbolic language should be taught explicitly, including its peculiarities such as "the function $f(x)$". (You may want to deprecate this usage when you teach it, but students deserve to understand its meaning.)
The positive integers
You have a mental representation of the positive integers $1,2,3,\ldots$. In this discussion I will assume that "you" know a certain amount of math. Nonmathematicians may have very different mental representations of the integers.
 You have a concept of "an integer" in some operational way as an abstract object.
 "Abstract object" needs a post of its own. Meanwhile see Mathematical Objects (abstractmath) and the Wikipedia articles on Mathematical objects and Abstract objects.
 You have a connection in your brain between the concept of integer and the concept of listing things in order, numbering them by $1,2,3,\ldots$.
 You have a connection in your brain between the concept of an integer and the concept of counting a finite number of objects. But then you need zero!
 You understand how to represent an integer using the decimal representation, and perhaps representations to other bases as well.
 Your mental image has the integer "$42"$ connected to but not the same as the decimal representation "42". This is not true of many students.
 The decimal rep has a picture of the string "42" associated to it, and of course the picture of the string may come up when you think of the integer $42$ as well (it does for me — it is a an icon for the number $42$.)
 You have a concept of the set of integers.
 Students need to be told that by convention "the set of integers" means the set of all integers. This particularly applies to students whose native language does not have articles, but American students have trouble with this, too.
 Your concept of "the set of integers" may have the icon "$\mathbb{N}$" associated with it. If you are a mathematician, the icon and the concept of the set of integers are associated with each other but not identified with each other.
 For me, at least, the concept "set of integers" is mentally connected to each integer by the "element of" relation. (See third bullet below.)
 You have a mental representation of the fact that the set of integers is infinite.
 This does not mean that your brain contains an infinite number of objects, but that you have a representation of infinity as a concept, it is brainconnected to the concept of the set of integers, and also perhaps to a proof of the fact that $\mathbb{N}$ is infinite.
 In particular, the idea that the set of integers is mentally connected to each integer does not mean that the whole infinite number of integers is attached in your brain to the concept of the set of integers. Rather, the idea is a predicate in your brain. When it is connected to "$42$", it says "yes". To "$\pi$" it says "No".
 Philosophers worry about the concept of completed infinity. It exists as a concept in your brain that interacts as a meme with concepts in other mathematicians' brains. In that way, and in that way only (as far as I am concerned) it is a physical object, in particular an object that exists in scattered physical form in a social network.
Graph of a function
This is a graph of the function $y=x^3x$:
 The graph is a physical object, either on a screen or on paper
 It is processed by your visual system, the most powerful sensory management system in your brain
 It also represents the graph in the mathematical sense (set of ordered pairs) of the function $y=x^3x$
 Both the mathematical graph and the physical graph are represented by modules in your brain, which associates the two of them with each other by a conceptual metaphor.
 The graph shows some properties of the function: inflection point, going off to infinity in a specific way, and so on.
 These properties are made apparent (if you are knowledgeable) by means of the powerful pattern recognition system in your brain. You see them much more quickly than you can discover them by calculation.
 These properties are not proved by the graph. Nevertheless, the graph communicates information: for example, it suggests that you can prove that there is an inflection point near $(0,0)$.
 The graph does not determine or define the function: It is inaccurate and it does not (cannot) show all of the graph.
 More subtle details about this graph are discussed in my post Representations 2.
Continuity
Example (C1) The $\epsilon\delta$ definition of the continuity of a function $f:\mathbb{R}\to\mathbb{R}$ may be given in the symbolic language of math:
A function $f$ is continuous at a number $c$ if \[\forall\epsilon(\epsilon\gt0\implies(\forall x(\exists\delta(xc\lt\delta\impliesf(x)f(c)\lt\epsilon)))\]
 To understand (C1), you must be familiar with the notation of first order logic. For most students, getting the notation right is quite a bit of work.
 You must also understand the concepts, rules and semantics of first order logic.
 Even if you are familiar with all that, continuity is still a difficult concept to understand.
 This statement does show that the concept is logically complicated. I don't see how it gives any other intuition about the concept.
Example (C2) The definition of continuity can also be represented in mathematical English like this:
A function $f$ is continuous at a number $c$ if for any $\epsilon\gt0$ and for any $x$ there is a $\delta$ such that if $xc\lt\delta$, then $f(x)f(c)\lt\epsilon$.
 This definition doesn't give any more intuition that (C1) does.
 It is easier to read that (C1) for most math students, but it still requires intimate familiarity with the quirks of math English.
 The fact that "continuous" is in boldface signals that this is a definition. This is a convention.
 The phrase "For any $\epsilon\gt0$" contains an unmarked parenthetic insertion that makes it grammatically incoherent. It could be translated as: "For any $\epsilon$ that is greater than $0$". Most math majors eventually understand such things subconsciously. This usage is very common.
 Unless it is explicitly pointed out, most students won't notice that if you change the phrase "for any $x$ there is a $\delta$" to "there is a $\delta$ for any $x$" the result means something quite different. Cauchy never caught onto this.
 In both (C1) and (C2), the "if" in the phrase "A function $f$ is continuous at a number $c$ if…" means "if and only if" because it is in a definition. Students rarely see this pointed out explicitly.
Example (C3) The definition of continuity can be given in a formally defined first order logical theory.
 The theory would have to contain function symbols and axioms expressing the algebra of real numbers as an ordered field.
 I don't know that such a definition has ever been given, but there are various semiautomated and automated theoremproving systems (which I know little about) that might be able to state such a definition. I would appreciate information about this.
 Such a definition would make the property of continuity a mathematical object.
 An automated theoremproving system might be able to prove that $x^3x$ is continuous, but I wonder if the resulting proof would aid your intuition much.
Example (C4) A function from one topological space to another is continuous if the inverse of every open set in the codomain is an open set in the domain.
 This definition is stated in mathematical English.
 All definitions start with primitive data.
 In definitions (C1) – (C3), the primitive data are real numbers and the statement uses properties of an ordered field.
 In (C4), the data are real numbers and the arithmetic operations of a topological field, along with the open sets of the field. The ordering is not mentioned.
 This shows that a definition need not mention some important aspects of the structure.
 One marvelous example of this is that a partition of a set and an equivalence relation on a set are based on essentially disjoint sets of data, but they define exactly the same type of structure.
Example (C4) "The graph of a continuous function can be drawn without picking up the chalk".
 This is a metaphor that associates an action with the graph.
 It is incorrect: The graphs of some continuous functions cannot be drawn. For example, the function $x\mapsto x^2\sin(1/x)$ is continuous on the interval $[1,1]$ but cannot be drawn at $x=0$.
 Generally speaking, if the function can be drawn then it can be drawn without picking up the chalk, so the metaphor provides a useful insight, and it provides an entry into consciousnessraising examples like the one in the preceding bullet.
References
 1.000… and .999… (post)

Concept Image and Concept Definition in Mathematics with particular reference to Limits and Continuity, David Tall and Schlomo Vinner, 1981
 Conceptual blending (post)
 Conceptual blending (Wikipedia)
 Conceptual metaphors (Wikipedia)
 Convention (abstractmath)
 Definitions (abstractmath)
 Embodied cognition (Wikipedia)
 Handbook of mathematical discourse (see articles on conceptual blend, mental representation, representation, metaphor, parenthetic assertion)
 Images and Metaphors (abstractmath).
 The interplay of text, symbols and graphics in math education, Lin Hammill
 Math and the modules of the mind (post)
 Mathematical discourse: Language, symbolism and visual images, K. L. O’Halloran.
 Mathematical objects (abmath)
 Mathematical objects (Wikipedia)
 Mathematical objects are “out there?” (post)
 Metaphors in computing science (post)
 Procept (Wikipedia)
 Representations 2 (post)
 Representations and models (abstractmath)
 Representations II: dry bones (post)
 Representation theorems (Wikipedia) Concrete representations of abstractly defined objects.
 Representation theory (Wikipedia) Linear representations of algebraic structures.
 Semiotics, symbols and mathematical visualization, Norma Presmeg, 2006.
 The transition to formal thinking in mathematics, David Tall, 2010
 Theory in mathematical logic (Wikipedia)
 What is the object of the encapsulation of a process? Tall et al., 2000.
 Where mathematics comes from, by George Lakoff and Rafael Núñez, Basic Books, 2000.
 Where mathematics comes from (Wikipedia) This is a review of the preceding book. It is a permanent link to the version of 04:23, 25 October 2012. The review is opinionated, partly wrong, not well written and does not fit the requirements of a Wikipedia entry. I recommend it anyway; it is well worth reading. It contains links to three other reviews.
Notes on Viewing
This post uses MathJax. If you see mathematical expressions with dollar signs around them, or badly formatted formulas, try refreshing the screen. Sometimes you have to do it two or three times.
I like thinking about 'presentations' versus 'representations', and what the similarity of those words suggests.
Juggling with alternative representations of the same problem has been the main research interest of Paul Benjamin, his 1992 paper "Towards an Effective Theory of Reformulation" highlight his approach of using semigroups over the problem representation.
Unfortunately its seems that, in the long run, this didn't amount to much and the approach petered out.
The above paper though not on his home page is still available as a NASA tech report:
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19960047149_1996049778.pdf