Permutation takes into account the order while in combination order is irrelevant. The relation between the two is nCr=nPr/r!. For example, if we select 5 objects out of 8 different objects, there are 120 different ways of arranging 5 objects (120=5!=5*4*3*2*1). In selecting 5 objects out of 8 we actually calculate 8*7*6*5*4=6,720 (8P5) which is a permutation when the order matters, but it's 6,720/120=56 combinations (8C5) if order doesn't matter.