Combinatorics is a branch of mathematics that deals with problems in selecting, arranging, and combining objects chosen from a given set.

To take simple examples, questions such as how many ways can a line of five people be ordered, or how many possible hands there are in a game of bridge, or how many ways there are to choose the six numbers in a lottery that uses the numbers from 1 to 49, fall under the field of combinatorics.

Also available at Math Lair is a Combinatorics Worksheet.