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Short for Universal Serial Bus, an external bus standard that supports data transfer rates of 12 Mbps. A single USB port can be used to connect up to 127 peripheral devices, such as mice, modems, and keyboards. USB also supports Plug-and-Play installation and hot plugging. Starting in 1996, a few computer manufacturers started including USB support in their new machines. It wasn't until the release of the best-selling iMac in 1998 that USB became widespread. It is expected to completely replace serial and parallel ports. Also see USB 2.0.
A typical host PC might have two USB 2.0 ports and two USB 3.0 ports. The USB 2.0 ports might be used for a USB keyboard and USB mouse, but the user may have far more than two additional USB devices to connect to the PC at the same time, and many of those might be capable of USB 3.0 operation.