TCPIP (Transfer Control Protocol / Internet Protocol) is the underlying protocol of the Internet today. It is the protocol that helps to split messages up, helps them find their way to the correct destination, then causes them rejoin at the destination. TCPIP is very important, but you rarely interact with it directly. All the other protocols take advantage of it.
TCPIP has been with us since the earliest days of the ARPANet. It is showing some signs of age, since it is being used for purposes its designers never imagined! Some other approaches to underlying Internet protocols are being studied. One notable possibility is a concept called ATM (Nope, not Automatic Teller Machines, but Asynchronous Transfer Mode.)