TCP/IP is a universal standard suite of protocols used to provide connectivity between networked devices. It is part of the larger OSI model upon which most data communications is based.
One component of TCP/IP is the Internet Protocol (IP) which is responsible for ensuring that data is transferred between two addresses without being corrupted.
For manageability, the data is usually split into multiple pieces or packets each with its own error detection bytes in the control section or header of the packet. The remote computer then receives the packets and reassembles the data and checks for errors. It then passes the data to the program that expects to receive it.
How does the computer know what program needs the data? Each IP packet also contains a piece of information in its header called the type field. This informs the computer receiving the data about the type of layer 4 transportation mechanism being used.
The two most popular transportation mechanisms used on the Internet are Transmission Control Protocol (TCP) and User Datagram Protocol (UDP).
When the type of transport protocol has been determined, the TCP/UDP header is then inspected for the "port" value, which is used to determine which network application on the computer should process the data. This is explained in more detail later.