UDP is a connectionless, best-effort Internet protocol that offers fewer features than TCP (for example, no handshaking, flow control or reliability mechanisms), in exchange for speedier delivery of data.
This makes it useful for applications in which the data must arrive quickly at an endpoint, such as videoconferencing and streaming media. In fact, the multimedia protocol SIP relies on UDP (although it adds its own retransmission mechanism since UDP is an unreliable protocol).
SNMP and DNS also use UDP.
UDP packets, or datagrams (also known as datagrams) contain, in addition to the lower level headers, a checksum, the packet length, source and destination ports.
Additional resources
Connectionless Transport: UDP
More details.
