Unity is a shell interface for the GNOME desktop environment developed by Canonical Ltd for its Ubuntu operating system. Unity debuted in the netbook edition of Ubuntu 10.10. It was initially designed to make more efficient use of space given the limited screen size of netbooks, including, for example, a vertical application switcher called the launcher.
Unlike GNOME, KDE Software Compilation, Xfce, or LXDE, Unity is not a collection of applications but is designed to use existing programs.
Unity is part of the Ayatana project, an initiative to improve the user experience within Ubuntu. In addition to Unity, there are Application Indicators and other projects such as MeMenu, the notification system and the application NotifyOSD gathered.
(Text and image from the wikipedia article)