Swing is a user interface toolkit - buttons, tables, and trees (oh my!).
Now, I'll admit that I'm just a tad biased - I'm one of the development engineers on the project at Sun - but one of the spiffiest things about Swing is that since the source code is fully available, and the toolkit is entirely written in Java, you can easily extend/modify the base components to your heart's desire.
Oh, also check out The Swing Connection for lots of in-depth articles written by the the engineers who wrote the toolkit.
http://java.sun.com/products/jfc/tsc
jeff