When you start GAP it already knows several groups. Currently GAP initially knows the following groups: item some basic groups, such as cyclic groups or symmetric groups (see The Basic Groups Library), The Primitive Groups Library), The Transitive Groups Library), The Solvable Groups Library), item the 2-groups of size at most 256 (see The 2-Groups Library), item the 3-groups of size at most 729 (see The 3-Groups Library), item the irreducible solvable subgroups of GL(n,p) for n > 1 and The Irreducible Solvable Linear Groups Library), item the finite perfect groups of size at most 10^6 (excluding 11 sizes) (see The Library of Finite Perfect Groups), item the irreducible maximal finite integral matrix groups of dimension Irreducible Maximal Finite Integral Matrix Groups), The Crystallographic Groups Library). item the groups of order at most 1000 except for 512 and 768 (see The Small Groups Library).
Each of the set of groups above is called a library. The whole set of groups that GAP knows initially is called the GAP collection of group libraries. There is usually no relation between the groups in the different libraries.
Several of the libraries are accessed in a uniform manner. For each of these libraries there is a so called selection function that allows you to select the list of groups that satisfy given criterias from a library. The example function allows you to select one group that satisfies given criteria from the library. The low-level extraction function allows you to extract a single group from a library, using a simple Selection Functions, Example Functions, and Extraction Functions.
Note that a system administrator may choose to install all, or only a few, or even none of the libraries. So some of the libraries mentioned below may not be available on your installation.