The "Set Difference" is a concept in set theory that refers to the elements that belong to one set but not to another. If you have two sets, A and B, the set difference, denoted as A - B, includes all the elements in A that are not found in B. This operation helps in understanding the unique elements of a set. For example, if A = 1, 2, 3, 4 and B = 3, 4, 5, then the set difference A - B would be 1, 2. This means that 1 and 2 are in A but not in B, highlighting the unique elements of A.