Stands for "Universal Serial Bus." USB is the most common type of computer port used in today's computers. It can be used to connect keyboards, mice, game controllers, printers, scanners, digital cameras, and removable media drives, just to name a few. With the help of a few USB hubs, you can connect up to 127 peripherals to a single USB port and use them all at once (though that would require quite a bit of dexterity).
USB is also faster than older ports, such as serial and parallel ports. The USB 1.1 specification supports data transfer rates of up to 12Mb/sec and USB 2.0 has a maximum transfer rate of 480 Mbps. Though USB was introduced in 1997, the technology didn't really take off until the introduction of the Apple iMac (in late 1998) which used USB ports exclusively. It is somewhat ironic, considering USB was created and designed by Intel, Compaq, Digital, and IBM. Over the past few years, USB has become a widely-used cross-platform interface for both Macs and PCs.