Order usually matters in things like "how many different ways can you do something". Ex: people standing in line, different title positions (President, VP, treasurer...), people going through different rooms.
Combinations are then where it wouldnt matter. Things like the books you choose in a set (obviously if you pick Book A before Book B, it doesnt matter), how many people you can bring to a ballgame (doesnt matter if Matt, John, Bill went or Bill, John, Matt....same people!!)
Just some examples I thought off the top of my head!