The ancient Greeks were great storytellers. They were also deeply religious. They believed their gods and goddesses and other magical beings really existed. They believed these magical beings could interfere in their lives, to help them or to hurt them.
The ancient Greeks built temples to honor their gods. Each temple honored only one god. So there were many, many temples in ancient Greece.
The ancient Greeks told stories about their gods. These stories are called myths (short for mythology, or stories about gods.) Some myths were so good that they have been told over and over, and are still told today. Some are new myths about the ancient Greek mystical world.
The thing about myths is that the gods' adventures might change, depending upon the storyteller, but the personalities and powers of the gods remained consistent. Zeus was always the king of all the gods. His wife, Hera, was always jealous. Poseidon, his brother, always ruled the sea. His other brother, Hades, always ruled the Underworld. His sister Demeter ruled the harvest. Hestia, his other sister, ruled the home.
All the gods had magical powers, although not all the gods had the same powers. Whatever powers they had were consistent from story to story. For example, Zeus and only Zeus could throw lightning bolts. Some gods could shape-shift and turn themselves into animals, humans, whatever they wanted.
This magical world was a real world, at least it was to the ancient Greek people. It was a world full of bickering and fights and wars and compromise and fear and fun and punishment and love. Many myths were based on the fact that gods, like mortal men, could be punished or rewarded for their actions.