Imagine two computers connected with a cable, so they can communicate with each other. The question then becomes, how? How do they communicate? How do they know that the other one is there? How does it tell which data goes to which computer?
The answer for the Internet is the Transmission Control Protocol / Internet Protocol -- generally refered to as TCP/IP. These two protocols (along with truckloads of others, but I don't want to bore to death) govern the way data is sent back and forth on the internet.