A protocol is a set of logical rules that define how computers send and receive information. There are two types of protocols:

1. proprietary and
2. open.

Proprietary protocols are vendor-specific. For example, AppleTalk works on Macintosh computers. Open protocols are published specifications that anyone can use.
For example, TCP/IP, HTTP, and FTP work on UNIX, Windows-based computers, and Macintosh computers.

Protocols can be either connectionless or connection-oriented. Connectionless protocols like User Datagram Protocol (UDP) do not maintain their connection to the client. After data is transmitted, the client and server disconnect and no effort is made to verify the data arrived at its destination. Because of this, connectionless protocols are sometimes considered unreliable.
UDP and other types of connectionless protocols do have their benefits though. Because they bypass the acknowledgement/verify stages used in connection-oriented protocols they are fast and efficient. For example, suppose you email a co-worker to remind them about an upcoming conference. They might not reply with a confirmation. So, even though your method of delivering the message was fast, there's no guarantee the co-worker completely understood or even received your message. That is the essence of using a connectionless protocol.

A connection-oriented protocol establishes a connection to begin connection and acknowledge all transmitted data. TCP is an example of a connection-oriented protocol. After data has been transmitted, the client remains connected to the server, permitting verification that the data has arrived, and/or to permit particular packets to be re-sent. For this reason, TCP is technically referred to as a reliable connection.
In the next lesson, you will learn about TCP/IP protocols.