Category theory
In mathematics, category theory deals in an abstract way with mathematical structures and relationships between them. Categories now appear in most branches of mathematics and in some areas of theoretical computer science and mathematical physics, and have been a unifying notion. Categories were first introduced by Samuel Eilenberg and Saunders Mac Lane in 1942-1945, in connection with algebraic topology.
Category theory has several faces known not just to specialists, but to other mathematicians. "General abstract nonsense" refers, perhaps not entirely affectionately, to its high level of abstraction, compared to more classical branches of mathematics. Homological algebra is category theory in its aspect of organising and suggesting calculations in abstract algebra. Diagram chasing is a visual method of arguing with abstract 'arrows', and has appeared in a Hollywood film, as Jill Clayburgh proved the snake lemma (at the start of It's My Turn, 1980). Topos theory is a form of abstract sheaf theory, with geometric origins, and leads to ideas such as pointless topology.

# Background
The study of categories is an attempt to capture what is commonly found in various classes of related mathematical structures.
Consider the following example. The class Grp of groups consists of all objects having a "group structure". More precisely, Grp consists of all sets G endowed with a binary operation satisfying a certain set of axioms. One can proceed to prove theorems about groups by making logical deductions from the set of axioms. For example, it is immediately proved from the axioms that the identity element of a group is unique.
Instead of focusing merely on the individual objects (e.g. groups) possessing a given structure, category theory emphasizes the morphisms  the structure-preserving mappings  between these objects. It turns out that by studying these morphisms, we are able to learn more about the structure of the objects.  In the case of groups, the morphisms are the group homomorphisms. A group homomorphism between two groups "preserves the group structure" in a precise sense  it is a "process" taking one group to another, in a way that carries along information about the structure of the first group into the second group. The study of group homomorphisms then provides a tool for studying general properties of groups and consequences of the group axioms.
A similar type of investigation occurs in many mathematical theories, such as the study of continuous maps between topological spaces in topology and the study of diffeomorphisms in manifold theory.  The notion of a category is an axiomatic formulation of this idea of relating mathematical structures to the structure-preserving functions between them. A systematic study of categories then allows us to prove general results from the axioms of a category.
A category is itself a type of mathematical structure, so we can look for 'processes' which preserve this structure in some sense. Such a process is called a functor. It associates to every object of one category an object of another category; and to every morphism in the first category a morphism in the second. By studying categories and functors, we are not just studying a class of mathematical structures and the morphisms between them, we are studying the relationships between various classes of mathematical structure. This is a fundamental idea, which first surfaced in algebraic topology. Difficult topological questions can be translated into algebraic questions which are often easier to solve.  Basic constructions, such as the fundamental group of a topological space, can be expressed as functors in this way.
Constructions are often "naturally related", a vague notion at first sight. This leads to the clarifying concept of natural transformation, a way to "map" one functor to another. Many important constructions in mathematics can be studied in this context. 'Naturality' is a principle, like general covariance in physics, that cuts deeper than is initially apparent.

# Historical notes
In 1942-45, Samuel Eilenberg and Saunders Mac Lane were the first to introduce categories, functors, and natural transformations, as part of their work in topology, especially algebraic topology. Their work was an important part of the transition from intuitive and geometric homology to axiomatic homology theory. Eilenberg and Mac Lane later wrote that their goal was to understand natural transformations; in order to do that, functors had to be defined, which required categories.
Stanislaw Ulam, and some writing on his behalf, have claimed that related ideas were current in the late 1930s in Poland. Eilenberg was Polish and studied mathematics there in the 1930s. Category theory is also, in some sense, a continuation of Emmy Noether's (one of Mac Lane's teachers) work in formalizing abstract processes. Noether realized that in order to understand a type of mathematical structure, one needs to understand the processes preserving this structure. In order to achieve this understanding, Eilenberg and Mac Lane proposed an axiomatic formalization of the relation between structures and the processes preserving them.
The subsequent development of category theory was powered first by the computational needs of homological algebra; and later by the axiomatic needs of algebraic geometry, the field most resistant to being grounded in either axiomatic set theory or the Russell-Whitehead view of united foundations. General category theory, an extension of universal algebra having many new features allowing for semantic flexibility and higher-order logic, came later; it is now applied throughout mathematics.
Certain categories called topoi (singular topos) can even serve as an alternative to axiomatic set theory as a foundation of mathematics. These foundational applications of category theory have been worked out in fair detail as a basis for, and justification of, constructive mathematics. More recent efforts to introduce undergraduates to categories as a foundation for mathematics include Lawvere and Rosebrugh (2003) and Lawvere and Schanuel (1997).
Categorical logic is now a well-defined field based on type theory for intuitionistic logics, with applications in  functional programming and domain theory, where a cartesian closed category is taken as a non-syntactic description of a lambda calculus. At the very least, category theoretic language clarifies what exactly these related areas have in common (in some abstract sense).

# Categories, objects and morphisms
A category C consists of the following three mathematical entities:
- A class ob(C) of objects;
- A class hom(C) of morphisms. Each morphism f has a unique source object a and target object b. We write f: a  b, and we say "f is a morphism from a to b". We write hom(a, b)  to denote the hom-class of all morphisms from a to b. (Some authors write Mor(a, b) or C(a, b).)
- A binary operation \circ, called composition of morphisms, such that for any three objects a, b, and c, we have hom(a, b)  hom(b, c)  hom(a, c). The composition of f: a  b and g: b  c is written as g\circ f or gf (some authors write fg), governed by two axioms:
- Associativity: If f : a  b, g : b  c and h : c  d then h\circ(g\circ f)=(h\circ g)\circ f, and
- Identity: For every object x, there exists a morphism 1x : x  x called the identity morphism for x, such that for every morphism f : a  b, we have {\rm 1}_b\circ f=f=f\circ{\rm 1}_a.
From these axioms, it can be proved that there is exactly one identity morphism for every object. Some authors deviate from the definition just given by identifying each object with its identity morphism.
Relations among morphisms (such as fg = h) are often depicted using commutative diagrams, with "points" (corners) representing objects and "arrows" representing morphisms. The influence of commutative diagrams has been such that "arrow" and morphism are now synonymous.

## Properties of morphisms
Some morphisms have important properties. A morphism f : a  b is:
- a monomorphism (or monic) if fog1 = fog2 implies g1 = g2 for all morphisms g1, g2 : x  a.
- an epimorphism (or epic) if g1of = g2of implies g1 = g2 for all morphisms g1, g2 : b  x.
- an isomorphism if there exists a morphism g : b  a with fog = 1b and gof = 1a.
- an endomorphism if a = b. end(a) denotes the class of endomorphisms of a.
- an automorphism if f is both an endomorphism and an isomorphism. aut(a) denotes the class of automorphisms of a.

# Functors
Functors are structure-preserving maps between categories. They can be thought of as morphisms in the category of all (small) categories.
A (covariant) functor F from a category C to a category D, written F:C  D, consists of:
- for each object x in C, an object F(x) in D; and
- for each morphism f : x  y in C, a morphism F(f) : F(x)  F(y),
such that the following two properties hold:
- For every object x in C, \ F(1_x) = 1_{F(x)};
- For all morphisms f : x  y and g : y  z, F(g\circ f)=F(g)\circ F(f).
A contravariant functor F: C  D, is like a covariant functor, except that it "turns morphisms around" ("reverses all the arrows"). More specifically, every morphism f : x  y in C must be assigned to a morphism  F(f) : F(y)  F(x) in D. In other words, a contravariant functor is a covariant functor from the opposite category Cop to D.

# Natural transformations and isomorphisms
A natural transformation is a relation between two functors. Functors often describe "natural constructions" and natural transformations then describe "natural homomorphisms" between two such constructions. Sometimes two quite different constructions yield "the same" result; this is expressed by a natural isomorphism between the two functors.
If F and G are (covariant) functors between the categories C and D, then a natural transformation from F to G associates to every object x in C a morphism x : F(x)  G(x) in D such that for every morphism f : x  y in C, we have y o F(f) = G(f) o x; this means that the following diagram is commutative:
The two functors F and G are called naturally isomorphic if there exists a natural transformation from F to G such that x is an isomorphism for every object x in C.

# Universal constructions, limits, and colimits
Using the language of category theory, many areas of mathematical study can be cast into appropriate categories, such as the categories of all sets, groups, topologies, and so on. These categories surely have some objects that are "special" in a certain way, such as the empty set or the product of two topologies. Yet, in the definition of a category, objects are considered to be atomic; i.e., we do not know whether an object A is a set, a topology, or any other abstract concept. Hence, the challenge is to define special objects without referring to the internal structure of these objects. But how can we define the empty set without referring to elements, or the product topology without referring to open sets?
The solution is to characterize these objects in terms of their relations to other objects, as given by the morphisms of the respective categories. Thus the task is to find universal properties that uniquely determine the objects of interest. Indeed, it turns out that numerous important constructions can be described in a purely categorical way. The central concept which is needed for this purpose is called categorical limit, and can be dualized to yield the notion of a colimit.

# Equivalent categories
It is a natural question to ask, under which conditions two categories can be considered to be "essentially the same", in the sense that theorems about one category can readily be transformed into theorems about the other category. The major tool one employs to describe such a situation is called equivalence of categories. It is given by appropriate functors between two categories. Categorical equivalence has found numerous applications in mathematics.

# Further concepts and results
The definitions of categories and functors provide only the very basics of categorical algebra. Additional important topics are listed below. Although there are strong interrelations between all of these topics, the given order can be considered as a guideline for further reading.
- The functor category DC has as objects the functors from C to D and as morphisms the natural transformations of such functors. The Yoneda lemma is one of the most famous basic results of category theory; it describes representable functors in functor categories.
- Duality: Every statement, theorem, or definition in category theory has a dual which is essentially obtained by "reversing all the arrows". If one statement is true in a category C then its dual will be true in the dual category Cop. This duality, which is transparent at the level of category theory, is often obscured in applications and can lead to surprising relationships.
- Adjoint functors: A functor can be left (or right) adjoint to another functor that maps in the opposite direction. Such a pair of adjoint functors typically arises from a construction defined by a universal property; it can be seen as a more abstract and powerful view on universal properties.

# Higher-dimensional categories
Many of the above concepts, especially equivalence of categories, adjoint functor pairs, and functor categories, can be situated into the context of higher-dimensional categories. Briefly, if we consider a morphism between two objects as a "process taking us from one object to another", then higher-dimensional categories allow us to profitably generalize this by considering "higher-dimensional processes".
For example, a (strict) 2-category is a category together with "morphisms between morphisms", i.e. processes which allow us to transform one morphism into another. We can then "compose" these "bimorphisms" both horizontally and vertically, and we require a 2-dimensional "exchange law" to hold, relating the two composition laws. In this context, the standard example is Cat, the 2-category of all (small) categories, and in this example, bimorphisms of morphisms are simply natural transformations of morphisms in the usual sense. Another basic example is to consider a 2-category with a single objectthese are essentially monoidal categories. Bicategories are a weaker notion of 2-dimensional categories where the composition of morphisms is not strictly associative, but only associative "up to" an isomorphism.
This process can be extended for all natural numbers n, and these are called n-categories. There is even a notion of -category corresponding to the ordinal number . For a conversational introduction to these ideas, see Baez (1996).