Its a program that allows you to share a virtual desktop between two different computer systems.
With one mouse and keyboard, you are able to get a handle on two computer systems, and also go your mouse from a single desktop to another. It is just like the computer systems are welded together. Ill demonstrate. This is just what my desk at home appears like when my desk is clean:
From the monitor that is left-hand have actually a or windows 7 system running. In the right-hand monitor we run Ubuntu 7.04 (Feisty Fawn). It shows up on the Ubuntu (right) monitor and vice-versa if I move my mouse off my Windows (left) monitor to the right. Plus cut-and-paste works between your machines too.