Lining up for the photo
Five friends can line up for a photo in 5×4×3×2×1 = 120 different orders. Counting arrangements is the first tool of probability — you can't find a chance without counting the possibilities.
Order matters or not
Permutations count ordered arrangements (n!). Combinations count unordered choices, C(n,k) = n!/(k!(n−k)!).