An external bus standard that supports data transfer rates of 12 Mbps (12 million bits per second). One USB port can connect up to 127 peripheral devices, such as keyboards, modems, soundcards etc.
USB Soundcards can be used for PC / Laptop / Mac and are usually installed simply by plugging straight into the USB port of your computer and clicking through the setup process.
USB works well for the data transfer of audio and midi but does have a limit. This is usually when working with 4 inputs and 4 outputs. The new standard of USB 2.0 has improved the data transfer to 480 Mbps, therefore enabling larger audio interfaces (soundcards) to work via USB.