USB is an Acronym for Universal Serial Bus.

This is a Bus with which peripherals such as mice, keyboards, scanners, KeyDrives, Scanners, Printers, and DigitalCameras etc can be to attached to a computer. USB provides power from the host computer. It also allows devices to be "chained". As a result, instead of having a mess of cables into the back of your computer, you could theoretically have your monitor and printer plugged into the computer, your scanner into the printer, your keyboard and camera into the monitor, and your mouse into the keyboard. So far, this scenario is purely theoretical.

One advantage USB has over some Legacy protocols such as PS/2 is the ability to plug devices in while the system is powered on - see also HotPlug.