You can read about here and here, but the easiest way to understand some of it's capabilities is to search for Compiz Fusion on YouTube.
It has a lot of cool effects that are fun to play with, and has some features that have the potential to improve productivity - particularly the window manipulation tools that allow you to scroll through multiple windows, or put them on a cube to rotate and open up, or treat them like pieces of paper that you can shuffle through and flip over. You can have 4 desktops that are quickly accessible, by flipping or scrolling.
Compiz Fusion comes with the Ubuntu Linux distribution.
These two YouTube videos show some Compiz Fusion capabilities -
