Communication among computers is accomplished by using protocols.
Protocols are essentially rules that computers agree to use when communicating.
A good analogy would be to think of computers as being able to speak many different languages. If one computer contacts another they must agree to speak the same language. If one is speaking Spanish and the other is speaking German, then it will not be a very productive conversation. The key to understanding protocols is to realize that protocols do not operate independently of each other but are stacked or layered on top of each other. For example, most data on the Internet travels in IP packets.