This week's book giveaway is in the Testing forum. We're giving away four copies of The Way of the Web Tester: A Beginner's Guide to Automating Tests and have Jonathan Rasmusson on-line! See this thread for details.
Iterator is newer than Enumeration, and its method names are shorter because it was felt that the old ones were too long and awkward. Also, Iterator has an optional method that lets you remove the current element from a collection -- Enumeration doesn't have that.
In any case, Iterator is meant as a replacement for Enumeration. You shouldn't use Enumeration unless you're forced to by a legacy API.