A locally presentable category is a category which contains a small set $S$ of small objects such that every object is a nice colimit over objects in this set.
This says equivalently that a presentable category $\mathcal{C}$ is a reflective localization $\mathcal{C} \hookrightarrow PSh(S)$ of a category of presheaves over $S$. Since here $PSh(S)$ is the free colimit completion of $S$ and the localization imposes relations, this is a presentation of $\mathcal{C}$ by generators and relations, hence the name (locally) presentable category.
See also at locally presentable categories - introduction.
There are many equivalent characterizations of locally presentable categories. The following is one of the most intuitive, equivalent characterizations are discussed below.
(locally presentable category)
A category $\mathcal{C}$ is called locally presentable if
it is an accessible category;
it has all small colimits.
This means
$\mathcal{C}$ is a locally small category;
$\mathcal{C}$ has all small colimits;
there exists a small set $S \hookrightarrow Obj(\mathcal{C})$ of $\lambda$-small objects that generates $\mathcal{C}$ under $\lambda$-filtered colimits
(meaning that every object of $\mathcal{C}$ may be written as a colimit over a diagram with objects in $S$);
every object in $\mathcal{C}$ is a small object (assuming 3, this is equivalent to the assertion that every object in $S$ is small).
The locally in locally presentable category refers to the fact that it is the objects that are presentable, not the category as such.
For instance, consider the notion of “locally finitely presentable category”, def. 3 below, in which the generating set $S$ consists of finitely presentable objects, i.e. $\omega$-small ones. If one dropped the word “locally” then one would get the notion “finitely presentable category” which means something completely different, namely a finitely presentable ($\omega$-small) object of Cat.
Since a small object is one which is $\kappa$-compact for some $\kappa$, and any $\kappa$-compact object is also $\lambda$-compact for any $\lambda\gt\kappa$, it follows that there exists some $\kappa$ such that every object of the colimit-generating set $S$ is $\kappa$-compact.
This provides a “stratification” of the class of locally presentable categories, as follows.
(locally $\kappa$-presentable category)
For $\kappa$ a regular cardinal, a locally $\kappa$-presentable category is a locally presentable category, def. 1, such that the colimit-generating set $S$ may be taken to consist of $\kappa$-compact objects.
Thus, a locally presentable category is one which is locally $\kappa$-presentable for some regular cardinal $\kappa$ (hence also for every $\lambda\gt\kappa$). In fact, in this case the fourth condition is redundant; once we know that there is a colimit-generating set consisting of $\kappa$-compact objects, it follows automatically that every object is $\lambda$-compact for some $\lambda$ (though there is no uniform upper bound on the required size of $\lambda$). Moreover, colimit-generation is also stronger than necessary; it suffices to have a strong generator consisting of small objects.
A locally ℵ${}_0$-presentable category is called a locally finitely presentable category.
There are various equivalent characterizations of locally presentable categories.
(as limit sketches)
Locally presentable categories are precisely the categories of models of limit-sketches.
This is (Adamek-Rosicky, corollary 1.52).
Restricted to locally finitely presentable categories this becomes:
Locally finitely presentable categories, def. 3, are equivalently the categories of finite limit preserving functors $C \to Set$, for small finitely complete categories $C$.
For the more detailed statement see below at Gabriel-Ulmer duality. Equivalently this says that:
Locally finitely presentable categories are equivalently models of finitary essentially algebraic theories.
(as accessible reflective subcategories of presheaves)
Locally presentable categories are precisely the accessibly embedded full reflective subcategories
of categories of presheaves on some category $K$.
This appears as (Adámek-Rosický, prop 1.46).
Here accessibly embedded means that $C \hookrightarrow Psh(K)$ is an accessible functor, which in turn means that $C$ is closed in $Psh(K)$ under $\kappa$-filtered colimits for some regular cardinal $\kappa$.
Write $Lex$ for the 2-category of small categories with finite limits, with finitely continuous (i.e., finite limit preserving) functors between them, and natural transformations between those.
Write $LFP$ for the 2-category of locally finitely presentable categories, def. 3, right adjoint functors which preserve filtered colimits, and natural transformations between them.
There is an equivalence of 2-categories
which sends a finitely complete category $C$ to the category of models of $C$, i.e., the category of left exact functors $C \to$ Set.
A slice category of a locally presentable category is again locally presentable.
This appears for instance as (Centazzo-Rosický-Vitale, remark 3).
If $A$ is locally presentable and $C$ is a small category, then the functor category $A^C$ is locally presentable.
We list examples of locally finitely presentable categories, def. 3.
The category Set of sets is locally finitely presentable.
For notice that every set is the directed colimit over the poset of all its finite subsets.
Moreover, a set $S \in Set$ is a $\kappa$-compact object precisely if it has cardinality $|S| \lt \kappa$. So all finite sets are ℵ$_0$-compact.
Hence a a set of generators that exhibits $Set$ as a locally finitely complete category is given by the set containing one finite set of cardinality $n \in \mathbb{N}$ for all $n$.
More generally, for $C$ any small category the category of presheaves $Set^C$ is locally finitely presentable if $C$ is small.
This follows with Gabriel-Ulmer duality: the finite limit completion of $C$, $Lex(C)$, is also small, and $Set^C$ is equivalent to the category of finitely continuous functors $Lex(C) \to Set$.
More generally still, if $A$ is locally finitely presentable and $C$ is small, then $A^C$ is locally finitely presentable.
To see this, embed $A$ as a finitely-accessible reflective subcategory of a presheaf topos $Set^B$, and then note that by 2-functoriality of $(-)^C$ we get $A^C$ as a finitely-accessible reflective subcategory of $Set^{B \times C}$.
The category of algebras of a Lawvere theory, for example Grp, is locally finitely presentable. A $T$-algebra $A$ is finitely presented if and only if the hom-functor $Alg_T(A, -)$ preserves filtered colimits, and any $T$-algebra can be expressed as a filtered colimit of finitely presented algebras.
The category of coalgebras over a field $k$ is locally finitely presentable; similarly the category of commutative coalgebras over $k$ is locally finitely presentable.
A poset, regarded as a category, is locally finitely presentable if it is a complete lattice which is algebraic (each element is a directed join of finite elements).
The category FinSet of finite sets is not locally finitely presentable, as it does not have all countable colimits.
The category of fields and field homomorphisms is not locally presentable, as it does not have all binary coproducts (for instance, there are none between fields of differing characteristics).
Top is not locally finitely presentable.
The opposite category of a locally presentable category (in particular, a locally finitely presentable category) is never locally presentable, unless it is a poset. This is Gabriel-Ulmer, Satz 7.13.
A poset, considered as a category, is locally presentable precisely if it is a complete lattice.
The following three examples, being presheaf categories, are locally finitely presentable, thus a fortiori locally presentable. They are important for the general study of (∞,1)-categories.
the category sSet of simplicial sets;
the category dSet of dendroidal sets.
for $C$ a small category the functor category $Funct(C,sSet)$ of simplicial presheaves.
More generally,
Every sheaf topos is locally presentable.
This appears for instance as (Borceux, prop. 3.4.16, page 220). It follows directly with prop. 3 and using that every sheaf topos is an accessibly embedded subtopos of a presheaf topos
The main ingredient of a direct proof is:
For $C$ a site and $\kappa$ a regular cardinal strictly larger than the cardinality of $Mor(C)$, every $\kappa$-filtered colimit in the sheaf topos $Sh(C)$ is computed objectwise.
This implies that all representables in a sheaf topos are $\kappa$-compact objects.
If $T$ is an accessible monad (a monad whose underlying functor is an accessible functor) on a locally presentable category $A$, then the category $A^T$ of algebras over the monad is locally presentable. In particular, if $A$ is locally presentable and $i: B \to A$ is a reflective subcategory, then $B$ is locally presentable if $i$ is accessible.
This appears in (Adamek-Rosicky, 2.78).
This is actually somewhat subtle and gets into some transfinite combinatorics, from what I can gather.
A combinatorial model category is a model category that is in particular a locally presentable category.
Given a class of morphisms $\Sigma$ in a locally presentable category, the answer to the orthogonal subcategory problem for $\Sigma^\perp$ is affirmative if $\Sigma$ is small, and is affirmative for any class $\Sigma$ assuming the large cardinal axiom known as Vopenka's principle.
See at Functor category – Local presentability.
Another notion of “presentable category” is that of an equationally presentable category.
Locally presentable categories are a special case of locally bounded categories.
Locally presentable categories: Cocomplete possibly-large categories generated under filtered colimits by small generators under small relations. Equivalently, accessible localizations of free cocompletions. Accessible categories omit the cocompleteness requirement; toposes add the requirement of a left exact localization.
The definition is due to
The standard textbook is
More details are in
Some further discussion is in
See also section A.1.1 of
where locally presentable categories are called just presentable categories.