A permutation describes one way to order a set of objects. The number of permutations is the number of distinct ways to order a set of objects, and can be calculated using the factorial. For example, the number of ways to arrange three letters $\text{A, B, C}$ is $3!$ or $3 * 2 * 1 = 6$. Enumerated, the permutations would include the set $\text{(A, B, C), (A, C, B), (B, A, C), (B, C, A), (C, A, B), (C, B, A)}$.