The Twelve Olympians are the most important gods of Greek mythology, who lived atop Mount Olympus. Seventeen gods have been said to have been Olympians, but there were never more than twelve at the same time.
Zeus, Hera, Poseidon, Demeter, Ares, Hermes, Hephaestus, Aphrodite, Athena, Apollo, and Artemis are always considered to be Olympians. Hestia, Dionysus, Hades, and Persephone are the gods who have sometimes been Olympians. Thus, Hestia gave up her place to Dionysus because she wanted to live amongst humans.
The Olympians became the most important gods when Zeus and his brothers and sisters won a war against the Titans. Zeus, Hera, Poseidon, Demeter, Hestia, and Hades are all brothers and sisters. All the other gods are Zeus's children and have different mothers.