What is the difference between set and hashset in C++ STL?

Question

When should I choose one over the other? Are there any pointers that you would recommend for using the right STL containers?

Answer 1

A hash_set would be implemented by a hash table, which has mostly O(1) operations, whereas a set is implemented by a tree of some sort (AVL, red black, etc.) which have O(log n) operations, but are in sorted order.

Edit: I had written that trees are O(n). That's completely wrong.