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.