Permutations and combinations
If rearranging the same selection makes a different result, it is a permutation; if not, it is a combination.
A worked example
Three medals among 7 runners: order matters, so 7P3 = 210. Three team members from 7: order does not, so 7C3 = 35.
The mistake people actually make
nCr is nPr divided by the r factorial orderings, so a combination answer is always smaller than the matching permutation. Ask whether swapping two of your picks changes anything before choosing.