HashSet and TreeSet are both implementations of the Set interface but differ in their characteristics. HashSet is an unordered collection that does not allow duplicate elements. It offers faster performance for adding, removing, and checking the existence of elements, as it internally uses hashing techniques. TreeSet, on the other hand, maintains elements in sorted order. It either follows the natural ordering of elements or uses a custom comparator. TreeSet is useful when elements need to be stored in a sorted manner, but it has slower performance compared to HashSet due to the overhead of maintaining the sorted order.