From Linux.com:

Unless you have fully embraced the virtualization movement, you probably have more than one machine in your home or office, particularly if you run more than one operating system, and you probably have more than one keyboard and mouse on your desk. If you would like to regain some desk space without having to purchase a KVM switch, x2x may be the solution. Simply put, x2x takes advantage of the X Window System’s ability to run a display over a TCP/IP network. In this case, you are not running a display as much as you are taking charge of another display’s mouse and keyboard.

More can be found here.