An unordered set works in a manner similar to a set, except that the internal behavior of these containers differs. A set makes use of red-black trees while an unordered set makes use of hash tables. The time complexity of set operations is O( log N) while the time complexity of unordered set operations is O(1); hence, the unordered set tends to be faster than the set.
The values stored in an unordered set are not organized in any particular fashion, unlike in a set, which stores values in a sorted fashion. If performance is the criteria, then an unordered set is a good bet; however, if iterating the values in a sorted fashion is a requirement, then set is a good choice.