This document discusses two types of combinations: combinations with repetition and combinations without repetition. Combinations with repetition allow elements to be chosen multiple times from a set, while combinations without repetition do not allow repeats. It provides examples to illustrate each type, such as how many 2-letter words can be created from 4 letters with repetition (10 combinations) versus without repetition (6 combinations). The key difference between the two types is whether repetition of elements is allowed or not when selecting elements from a set.