[GEEK} Good article about pluggable LAFs in Java
One of the hairier and most interesting parts of Swing (Java's GUI layer) are the so-called pluggable look and feels (or is it looks and feel?.) Truth is that you can live a very happy Swing life without knowing how it works, but if you are curious or need to know for some reason...
This document is a good summary, even though a little dated, if you are interested in really understanding it. Like many things it's not so complex once you understand it.