TCP is connection-oriented. Connection-oriented is a kind of connection established between the source & destination machines before any data is sent that means a connection is established & maintained until such time as the message or messages to be exchanged through the application programs at each end have been exchanged. The connections provided through TCP are called Virtual Connections. It means that there is no physical connection among the computers directly.
TCP is used along the Internet Protocol to send data in the form of message units among computers over the Internet. Whereas IP takes care of handling the real delivery of the data, TCP takes care of having track of the individual units of data (called Packet) which a message is divided into for efficient routing by the Internet. TCP provides for reliable, connection-oriented data transmission channel among two programs. Reliable means that data sent is definite to reach its destination in the order sent or an error will be returned to the sender.
For instance, while an HTML file is sent to someone from Web server, the Transmission Control Protocol (TCP) program layer in that server divides the file in one or more packets, numbers the packets, and after that forwards them individually. Though each of packets has the similar destination IP address, it might get differently routed through the network. At the other end (the client program in our computer), TCP reassembles the particular packets and waits till they have arrived to forward them as a single file.
TCP is responsible for making sure that a message is divided in the packets that IP manages and for reassembling the packets back into the complete message at the other end. In Open Systems Interconnection (OSI) communication model, TCP is in layer four, the Transport Layer.