UDP is an abbreviation of User Datagram Protocol.
TCP is the abbreviation for Transmission Control Protocol.
The main difference is that UDP messages are sent without checking whether the message arrives at anything. This makes it possible to send a message to a large group of recipients. This is in contrast to TCP messages, which are specifically sent to a receiver and can therefore also give an acknowledgement of receipt.
Both protocols are described in RFC 768 and are part of the basic protocols of the Internet.