People have always been interested in knowing how many ways there are to select or arrange a set of objects. A permutation of a number of objects is an arrangement of those objects in a specific order. A combination is a selection of objects considered without regard to their order. The difference between the two is that in a permutation, order counts, while in a combination, order does not count.
There are two worksheets about permutations and combinations available on Math Lair: A permutations worksheet and a combinations worksheet.