The Transmission Control Protocol (TCP) is a protocol of the Internet protocol suite (TCP/IP).
It originated in the initial network implementation in which it complemented the Internet Protocol (IP). Therefore, the entire suite is commonly referred to as TCP/IP.
It is a network layer protocol for one-to-one connections.
TCP ensures that messages between computers are transmitted:
A Message is a stream of octets (bytes) between applications running on hosts communicating via an IP network.
Socket communications are nowadays bidirectional.
HTTP runs over TCP.
Major internet applications rely on TCP: