TCP, the Transfer Control Protocol, takes a best-effort packet oriented protocol such as IP and provides reliable byte streams. This means that TCP will deal with acknowledging received packets, retransmitting lost packets and reorders packets received in the wrong order so that the receiver gets data in the same order it was sent. TCP is usually built over IP, hence the common term TCP/IP.
In addition, TCP adds ``ports'' which identify which allow multiple connections to be made to the same address. Ports below 1024 are typically reserved for the super-user or operating system. Common port numbers may be found in the file /etc/services on most Unix machines.