Well, all you really need to know is that they're real, whether you've heard them or not. There are thirteen major gods: Zeus, Hera, Poseidon, Hades, Demeter, Apollo and his twin, Artemis, Hephaestus, Hermes, Aphrodite, Athena, and Dionysus. Each governs a different area of reality. The most important are the Big Three: Zeus, King of the Gods and ruler of the sky, Poseidon, god of the seas and oceans, and Hades, ruler of the Underworld and Death. They're the most powerful.

The reason I mentioned them is because usually when the world is ending or something, they're involved, as well as their children.