Acronym for User Data Protocol. See UserDatagramProtocol

Basically multiuser IP, over the top of IP it adds the concepts of "ports". UDP is an unreliable service, packets may arrive out of order, not at all, or, in some cases corrupted1?.

|00|01|02|03|04|05|06|07|08|09|10|11|12|13|14|15|16|17|18|19|20|21|22|23|24|25|26|27|28|29|30|31 ||||||||||||||||||||||||||||||||^ IPv4 / IPv6 Header with protocol 0x11 (17) ||||||||||||||||Source Port||||||||||||||||Destination Port ||||||||||||||||Length||||||||||||||||Checksum ||||||||||||||||||||||||||||||||v Payload

1?: Yes, only if you disable the checksums, but some people do this per host, so you can't rely on it being set on your machine. Sigh.