A protocol is a set of rules governing the format of messages that are exchanged between computers, and enables them to communicate. Protocols are to computers what language is to people. To comprehend this text, you must understand English. Similarly, for two devices on a network to successfully communicate, they must both understand the same protocol. Applications or even computers that follow the same protocol can exchange data, even if they don't run the same operating system and are not made by the same vendor.

Protocols are often "layered" on top of other protocols, taking advantage of an existing communication infrastructure and adding additional capabilities.

Examples of protocols are: TCP/IP (Transmission Control Protocol/Internet Protocol), FTP (File Transfer Protocol), SMTP (Simple Mail Transfer Protocol), HTTP (Hypertext Transfer Protocol), etc.

OPC is not a protocol; rather it is based on the DCOM, or .Net APIs.

See also: API