TCP is the most used protocol of TCP/IP. While IP specifies the standards to let one computer find another computer, TCP specifies a standard, on top of IP, to let a computer connect to another computer. Once connected, the two computers can use a serial method to carry out a prolonged conversation.
To continue with our telephone analogy, TCP specifies how to carry on a sustained conversation over a phone: how to greet, how to talk, and how to hang up.