Displaylink is a technology that allows you to send monitor signal down a USB connection. It is recieved by a Displaylink hub that extracts the video signal and outputs it to a DVI, HDMI or VGA port that you connect to you monitor. It is usually used to drive an additional screen from a notebook computer.
I built a nice KVM solution for my two computers at home. My monitor has a USB hub built in so I plug my keyboard and mouse into that. Now to change the PC that I manage I simply plug the USB cable from my Pluggable Displaylink hub into the chosen PC. If I was too lazy to physically move the cable I could use use a cheap USB switch.