On April 28th, Ubuntu 11.04 (Natty Narwhal) will be released. 11.04 is slated to be the first Ubuntu release that will use Unity, in place of GNOME, as the default desktop shell.
What is Unity?
Unity is a shell interface for the GNOME desktop environment developed for Ubuntu. Ubuntu 11.04 will still be based on GNOME 3 – the underlying infrastructure, applications, etc will not change; Unity just defines what your desktop looks like (similar to Windows Explorer). Unity was originally designed for netbooks, but over the last few months has shown that it is suitable for desktops as well. Unity puts major emphasis on screen space; every pixel is utilized.
Unity is much more then just a few menu/window tweaks though. Unity is part of a larger project called Ayatana, which adds things like: