The UDP tracker protocol is a high-performance low-overhead BitTorrent tracker protocol. It uses the stateless User Datagram Protocol (UDP) for data transmission instead of the HTTP protocol (over TCP) regular trackers use. The data is in a custom binary format instead of the standard bencode algorithm BitTorrent uses for most communication.
URLs for this protocol have the following format: udp://tracker:port. This protocol is supported by only some BitTorrent clients.
Read more about UDP Tracker: Comparison With The HTTP Tracker, Clients Implementing The Protocol, Criticisms