I'm going to assume you're being serious. Excuse me that my Elder knowledge isn't complete. Cthulhu (pronounced shoo-thool-hu by the way) is one of the Elder Gods along with The Goat of a thousand young and that which man was not meant to know. basically something like 5 Billion years ago a race of ancient aliens colonised the Earth. When I say aliens I don't mean greys or anthromorphic creatures. Think something like 100 tentacles and even more eyes. I'm not sure, but Cthulhu is about 1 kilometer high (then again I might be wrong) and I once read of a game session where someone tried to attack it the reponse being "What, are you kidding. It's Cthulhu. He doesn't even have a hit point score", as it would be impossible to harm him. The writer (whose name I cannot at this time recall) must have been insane and continually on mind altering drugs, or at least thats what it appears like.
Edit: doh!. Why the heck am I showing my lack of knowledge by trying unsuccesfully to explain when I can link to info
http://www.cthulhu.org/cthulhu/