The ancient Greek mystical world was ruled by a small group of powerful gods called the Olympians. When things had to be decided, this council of 12 gods met on Mount Olympus to discuss things.
The 12 Olympian gods all kept a home on Mount Olympus. Unless they were off traveling somewhere, that's where you could find most of them. Hades preferred his home in the Underworld. Poseidon preferred his palace under the sea. But the rest of the Olympians lived on Mount Olympus year around.
Hestia was the goddess of hearth and home. She used to be one of the Olympians, but she grew tired of all their fighting and bickering. She gave her seat to Dionysus, the god of wine. Actually, once Dionysus settled down and got married, he was a very good choice. (But Hestia kept her home on Mount Olympus. The view was magnificent!)
Who were the 12 Olympians? When Hestia resigned, Dionysus took her place, so that was a swap. But no one knows whether Hades or Hephaestus was the 12th Olympian. It depended upon the storyteller - some said Hades, some insisted it was Aphrodite's husband, Hephaestus.
Here are the twelve Olympians: