Start with permutations: P(n,k) = n! / (n-k)! counts arrangements. But each selection of k items can be arranged in k! ways.
If you pick items A and B, permutations count both AB and BA. Combinations should count this only once.
Dividing by k! removes the duplicate orderings. C(n,k) = P(n,k) / k! = (n! / (n-k)!) / k! = n! / (k!(n-k)!).