Roo 1.0 has just been released and I'm chomping at the bit to do a project with it. Roo is a RAD tool for building Java/Spring web applications. It uses a command line code generator similar in style to Ruby on Rails. However, it is different to RoR in that is leaves you with pure Java code at at the end of the day. It also ties into Spring Tool Suite to provide integration with your eclipse IDE. So changes made in the IDE are picked up by the Roo daemon which effects related changes in associated Roo aspects and controllers. i.e. you can change your business logic and Roo will automatically make the changes need to reflect those changes in you controller/view layers.
The description probably best left up to the author Ben Alex. See his post about the release of Roo here:
Update: Spring Tool Suite is not required to use Roo, it just provides some enhancements. You can work in your existing IDE and Roo will pick your code changes.