USB DisplayLink monitors are a relatively new breed of peripheral for Macs and PCs. The technology creates a virtual display on your Mac/PC, compresses it, and then sends it over USB 2.0 to a monitor that uncompresses it. You would think this could cause a lag, but the transfer is almost un-noticeable by the naked eye. It is basically an extra monitor that connects to your USB port and frees your monitor port for other uses.
