This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
Lightweight components are those without peer windows in the host system. Heavyweight components, even simple things like buttons, each have a corresponding window in the underlying OS. This has implications for the capabilities and behavior of components. For instance, when you click on a heavyweight component, the window in the OS gets the click event and sends it to the VM. When you click on a lightweight component, the click falls through to the first heavyweight component (usually a JFrame), which must then decide which lightweight was actually clicked, and call its event handler. Another difference is that lightweight components may have transparent areas. It's true that some OS's windowing systems allow transparency, but the native code behind the AWT doesn't use it, so your only alternative for non-rectangular buttons and the like is to use lightweight components. Because they are implemented entirely in Java, lightweight components are far more flexible. You can control their appearance and event handling to a far greater degree than heavyweights, which depend on the native code that supports the AWT.