TCP/IP actually consists of a whole bunch of different protocols. At the lowest harware-independent layer (the network layer) is IP, which deals with getting packets of data from one machine to another and how to route them through other machines along the way, on a "best-effort" basis with no actual guarantee that they will make it to their destination. On top of this are built UDP and TCP, both of which are transport-layer protocols--they are how a program running on one machine can communicate with a program running on another machine. UDP, like the lower-level IP, makes no guarantees about reliable delivery or even that packets will arrive in the same order in which they were sent, whereas TCP is a full connection-oriented protocol which guarantees delivery and in the right order, or your money back.
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