The Lightweight X11 Desktop Environment (LXDE) resembles a classic Unix project -- it's partly constructed out of pre-existing programs, its emphasis is on speed, and its configuration requires taking time in a text editor. Even the relatively low quality of fonts on the desktop makes it feel like a vintage program. The result is a desktop environment that is short on innovation, but performs well on low-end machines, and blazingly fast on recent ones.

Rather than reinvent the desktop, LXDE relies by default on the window manager IceWM, the file manager PCManFM, and the image viewer GPicView. Each of these programs is known for speed, especially IceWM, which is frequently used by itself on low-end systems. However, you can also configure LXDE to use other window managers, if you choose.

Only three elements of LXDE are unique: LXPanel, LXSession, and LXMusic, the first and, so far, only utility for the desktop. An optional feature is the LXIce theme for IceWM, which comes in a bzipped file.

More here