UIKit is a Java library for creating user interfaces. Instead of hardcoding your entire user interface description directly in your source code, it allows you to create XML files which describe sets of components, their properties, their layouts, and their interactions with each other and with your program.
UIKit is not a replacement for AWT. It uses standard components, but gives you a better way of describing and instantiating them. It also is extremely flexible. You can easily create your own custom components, and include them in your layout definition files.