UDP stands for User Datagram protocol .UDP is different from TCP that it doesn’t provide reliability or the correct data sequence .The client send what is called Datagram to the client .These datagram’s may not arrive and may be arrive corrupted or also not in same order . You may ask why we need UDP. Why don’t we just use TCP to avoid these problems?

UDP there is no overhead of checking and correction that is exists in TCP .So in environment and application that need more speed regardless of the reliability like Video Streaming and Voice over IP. Most time sensitive application will prefer UDP because it is faster than TCP.

For example:
In the video conferences, if the quality is just good and in the same time speed is achieved .this is enough.
UDP is stateless protocol. It is so useful in cases where there is one server and many clients .You can make broadcast easy. (Broadcast means sending to many clients). (Ex: radio channel, one server multiple clients)

Datagram Congestion Control Protocol (DCCP).
In UDP there techniques use to avoid network congestion .One of theses techniques is DCCP. The DCCP Protocol is responsible for controlling the traffic congestion .