The Universal Serial Bus (USB) is a bus standard for connecting devices to a computer (usually a PC). Over a hundred devices can be connected and they can be inserted and removed whilst the computer is still powered on ("hot plugging and swapping").
So far, the only real uses of USB are peripherals, such as mice, keyboards, and digital cameras. There have been experiments that have monitors, hard drives, and networking components talking together, but these have not achieved general acceptence.
There are predictions that USB may replace PS/2 and AT style connections for mice and keyboards, but probably will not become a true "Universal Bus". Just like SCSI could have done everything, but turned out to excel at storage devices, USB may end up being used for a part of the interconnect on computers.
See also: Firewire