Cthulhu (pronounced kuh-THOO-loo or kuh-loo-oo) is a fictional monster created by the horror writer H. P. Lovecraft. Cthulhu is a god-like being from another planet, and it (Cthulhu is neither male nor female) is supposedly very ancient compared to humans. It has the alien appearance of being a large green dragon with the head of a squid. There is said to be many people that worship him as their deity because of his powers over the mind. His main element is water.
Cthulhu is one of the Great Old Ones, a group of god-like monsters from another place that now inhabit Earth, waiting to rise up and destroy the human race so they can rule the planet. The Old Ones are neither good nor evil, meaning that they do not hate humans, they just do not care about them. They think of humans as if they could be ants. Also, their appearance is so unlike anything of this world, that it is said that any human who so much as gazes upon one of them will go insane.
In one of Lovecraft's tales, The Call of Cthulhu, he said that Cthulhu currently lies in a dreamlike state in the underwater sunken city of R'lyeh (rill-ee-yeh), waiting for the time when the Old Ones shall rise again. R'lyeh is located in the Pacific Ocean (though it is not even a real city).