User Datagram Protocol- a protocol used for data transfering.  It is used in place of the TCP when the delivery is not needed to be reliable.  Video and audio traffic use UDP.  It is a connectionless protocol, meaning that it can send data without hooking up to a receiver. Most protocols require a sender and receiver connection.  The UDP is not reliable in that the message is sent and someone may or may not retrieve it.  It is useful in broadcasting- which simply means that more than one person may retrieve the data.

A few more specifics on UDP can be found HERE.