A combination is any subset of a particular set of objects, regardless of the order of selection. If the set consists of n objects, r objects can be selected giving n!/r!(n − r)! different combinations. This can be written nCr.
A permutation is an ordered subset (i.e. attention is paid to the order of selection or arrangement) of a particular set of objects. If the set consists of n objects, r such objects can be selected to give n!/(n − r)! permutations. This is written nPr.