UDP is a packet oriented network protocol (technically the User Datagram Protocol; datagrams are an older name for packets). This is the datagram partner to the stream oriented TCP. UDP is often called "unreliable". This only means that the user is responsible for handling dropped or corrupted packets. UDP is often used when the loss of a packet is relatively harmless. An example might be a "You have mail" server that sends UDP packets every 30 seconds when there is unread mail. When you find yourself adding code to account for dropped or corrupted packets or packets arriving out of order, it is a sure sign that you should be using TCP in the first place.

Some protocols based on UDP are

Also see “UDP for Tcl