Suppose you have 8 books at home and you want to make a selection of 4 out of those books for your vacations.
To decide if this situation involves permutations or combinations, we must recall that:
* A permutation is required when the order of appearance of the elements is important
* A combination is required when the order of appearance of the elements is not important
If you take 4 books at random from your 8 books available, you must ask yourself: Does the order in which I select them is important?
We usually don't bother if book A is after book B in our collection, so the common situation does not require us to bother with the order of appearance, so we should use combinations.