Platform UI provides the basic building blocks for user interfaces built with Eclipse. Some of these form the Eclipse Rich Client Platform (RCP) and can be used for arbitrary rich client applications, while others are specific to the Eclipse IDE. The Platform UI codebase is built on top of the Eclipse Standard Widget Toolkit (SWT), which is developed as an independent project.
For more information, refer to the Platform UI wiki page.
For information how to contribute to the Platform UI project see Platform UI - How to contribute wiki page.