Combinations
A combination is a mathematical technique that determines the number of possible arrangements in a collection of items where the order of the selection does not matter. In combinations, you can select the items in any order.
A k-combination of a set S is a subset of k distinct elements of S. If the set has n elements, the number of k-combinations is equal to the binomial coefficient nCk = n! / k! * (n - k)! whenever k <= n and which is zero when k > n. [Source: Wikipedia]
