HTTP: you see it every day, but do you know what it means? It stands for Hypertext Transfer Protocol. Wikipedia terms it “the foundation of data communication” for the web. It’s that, and impossible to illustrate.
Essentially, the protocol allows the various network elements between a client and a server to communicate with each other. Tim Berners-Lee (he of the WWW) and his team invented the original HTTP.
That’s probably enough for now, but if you’d like to know more here’s the Wikipedia entry.