Combinatorics is the branch of

mathematics that deals with determining the

number of

ways to

arrange things, the number of

subsets of a

set that have a certain property, etc.

The two fundamental concepts in combinatorics are permutations and combinations.

A permutation is an arrangement of a set of objects, or specifically an arrangement of a subset of a certain size of some set of objects.

A combination is a subset of a certain size of some set of objects, without regard to their order.

These concepts are inherent to some kinds of probability problems.