(Universal Serial Bus) A widely used hardware interface for attaching a maximum of 127 peripheral devices to a computer. There are usually at least two USB ports on laptops and four USB ports on desktop computers. After appearing on PCs in 1997, USB quickly became popular for connecting keyboards, mice, printers and external drives and eventually replaced the PC's serial and parallel ports.

USB devices are "hot swappable;" they can be plugged in and unplugged while the computer is on. This feature, combined with easy-to-reach ports on the front of the computer case, gave rise to the ubiquitous USB drive for backup and data transport. See USB drive.

USB 1.0, 2.0 and 3.0
USB 1.0 (1996) and USB 1.1 (1998) provide a Low-Speed 1.5 Mbps subchannel for keyboards and mice and a Full-Speed channel at 12 Mbps. Hi-Speed USB 2.0 (2001) jumps the top rate to 480 Mbps. SuperSpeed USB 3.0 (2008) provides a huge 10x increase over USB 2.0 to 4.8 Gbps (see USB 3.0).

Hubs Add Extra Ports
A USB hub plugs into one USB port on the computer and provides four or more shared ports for peripherals. Monitors and keyboards may also have built-in hubs, which offer convenient sockets on top of the desk for connecting devices. For more details, see USB hub. See PoweredUSB, USB device class, USB OTG, USB drive, USB printer, USB switch and USB toy.