A protocol for transmitting data. As contrasted to TCP, UDP is:

• packet-based
  Data is sent as discrete packets. No buffering is performed and packets are sent as soon as the sending application generates them.
• unreliable
  UDP has no mechanism to notify the sending application whether a packet was delivered succesfully or was lost in transmission.
• unordered
  No guarantees are made as to the order that data is presented to the receiving application.

See:

• RFC 768