What is a protocol?

Protocols enable different types of computers (Macintosh, PCs, UNIX, VMS, etc.) to communicate in spite of their differences. This is because they describe a standard format and method for communicating.

A very simple comparison can be made to Morse Code:

People using Morse Code agree that a given combination of dots and dashes will always have the same meaning.
Someone who speaks only Russian and someone who speaks only Greek can exchange messages using Morse Code because they agree on the format and method for exchanging messages.