For many years, the bulk of networking over IP has made use of just two protocols: transmission control protocol (TCP) and user datagram protocol (UDP). TCP offers a reliable, stream-oriented connection which works well for a large variety of higher-level network protocols. UDP, instead, makes a best effort to move individual packets from one host to another, but makes no promises regarding reliability or ordering. Most higher-level protocols are built upon TCP, but there are applications which are better served by UDP.
Link: Linux Weekly News
