SWT is an alternative to AWT and Swing, supported by the Eclipse community. Together with JFace it can be used to build RICH clients. SWT use native widgets, yet code is portable to other platforms. This is achieved by a JNI call-through to the OS library . Widgets are simply Components and JComponents in AWT/Swing, which includes text boxes, push buttons, etc.
JFace provides a higher-level API for SWT widgets, such as an ApplicationWindow or Tables and Trees backed with a data model. JFace also allows direct access to the underneath SWT widgets, so that developers can easily change whats beneath. The whole API is very exposed, in fact, even the threading model and event loop is totally exposed. Whatever thread that created the display becomes the GUI thread, and other resource extensive tasks have to be thread-spawned by the programmer hiimself. The good thing is SWTException is thrown if UI manipulation was not done in the GUI thread.
For existing Swing developers,  provides an extremely simple introduction, with several tables to show the relationship between Swing components and SWT Widgets. Also see  for a nice list of widget screenshots.