Definition: A protocol is a set of rules or agreed upon guidelines for communication. When communicating it is important to agree on how to do so. If one party speaks French and one German the communications will most likely fail. If they both agree on a single language communications will work.
On the Internet the set of communications protocols used is called TCP/IP. TCP/IP is actually a collection of various protocols that each have their own special funtion or purpose. These protocols have been established by international standards bodies and are used in almost all platforms and around the globe to ensure that all devices on the Internet can communicate successfully.