According to
Wikipedia:
JFace is defined by the Eclipse project as "a UI toolkit that provides helper classes for developing UI features that can be tedious to implement."[1] It is a layer that sits on top of the raw widget system, and provides classes for handling common UI programming tasks. It brings model view controller programming to the Standard Widget Toolkit.
To me that's just a hollow catch phrase though, so if anyone who has actually used it can clarify it a bit, please feel welcome.