permutations and combinations
Use permutations when ordering is important. For example take letters A, B, C.
So we can arrange them in ABC, ACB, CAB, CBA, BAC, BCA. So by counting we find there are 6 different ways to arrange three letters.
Use combinations when order is not important. For example, the A, B, C can have only one combination whether you call it ABC, or CBA, doesn't matter.
So keep in mind when solving any related problem, and note carefully whether order is important or not.