The Greeks imagined a big family of gods who lived on top of a mountain called Mount Olympus. The king of the gods was Zeus, who could throw thunderbolts. His brother Poseidon ruled the sea and could make storms or earthquakes. Their sister Athena was the goddess of wisdom. There were many more - Hermes the messenger with winged sandals, Apollo who looked after music, Artemis who hunted in the forests.
Then there are the heroes - the human (or half-human) characters who go on adventures. Hercules was famously strong and had to do twelve impossible tasks, like cleaning a whole stable in a day. Odysseus took ten years to sail home and met a one-eyed giant and a witch who turned his crew into pigs along the way. Theseus had to find his way out of a maze with a monster in it, using only a ball of string.
There are magical creatures too. Pegasus is a horse with feathery wings. The Minotaur is half-man, half-bull. The Phoenix is a bird that bursts into flames and is reborn from its own ashes. Medusa has snakes for hair, and looking at her turns you to stone.
These stories were told for hundreds of years before anyone wrote them down. People sat around fires at night and the older storytellers would act them out. Today, you can find them in books in any library - and you might recognise the names from cartoons, video games or films. Greek myths never really stop being told.