USB stands for Universal Serial Bus, a data transfer format that has become, well, universal in the past several years. The USB standard is used for everything from printers to hard drives, to lights, to flash memory units. The popularity of USB has made the technology inexpensive to the point where basically anyone can make a USB device.
USB comes in three versions: 1.0, 1.1, and 2.0. Modern implementations allow for things like power over the USB line, and faster data transfer rates. A popular trend in the PC market is to make USB 1.1 hardware and advertise it as "Hi-Speed" USB, which implies 2.0 compatibility and makes more money.