Difference between TreeSet and HashSet

The difference between these two sets is simply the order/algorithm in which items are added to/accessed/removed from the map. The circumstances determine which particular implementation is better/faster.

A treeSet organizes data in a tree through use of Comparator (natural ordering) and the hashSet organizes data in a hash table (using a hash function).
See the Java API for more info:

http://www.j2ee.me/javase/6/docs/api/java/util/TreeSet.html
http://java.sun.com/javase/6/docs/api/java/util/HashSet.html

The previous answer (unless I'm mistaken) is pretty much completely wrong...

Regards