[Xfce is a lightweight desktop environment for unix-like operating systems. It aims to be fast and lightweight, while still being visually appealing and easy to use.
Xfce 4 is a complete rewrite of the previous version. It's based on the GTK+ toolkit version 2. Xfce 4 has a radically different architecture from Xfce 3. It embodies the traditional UNIX philosophy of modularity and re-usability. All of Xfce 4's core components have been written from scratch in order to fit into the new architecture.
Another priority of Xfce 4 is adhereance to standards, specifically those defined at freedesktop.org. This allows Xfce 4 to seamlessly interoperate with programs written for other desktop environments, if those programs follow the specified standards.
Xfce 4 consists of a number of components that together provide the full functionality of the desktop environment. They are packaged separately and you can pick and choose from the available packages to create the best personal working environment.
Xfce is all free software. The separate components are released under either the BSD license, the GNU LGPL or the GNU GPL (see the section called Copyright and Licences for details).