Greek Gods

In Greek mythology, the Twelve Olympians, also known as the Dodekatheon (Greek: Δωδεκάθεον < δώδεκα, dōdeka, "twelve"+ θεοί, theoi, "gods"), were the principal deities of the Greek pantheon, residing atop a mythical Mount Olympus. The Olympians gained their supremacy in a war of gods in which Zeus led his siblings to victory over the Titans.

The concept of the "Twelve Gods" is older than any extant Greek or Roman sources, and is likely of Anatolian origin. The gods meet in council in the Homeric epics, but the first ancient reference to religious ceremonies for the Olympians collectively is found in the Homeric Hymn to Hermes. The Greek cult of the Twelve Olympians can be traced to 6th-century BC Athens and probably has no precedent in the Mycenaean period. The altar to the Twelve Olympians at Athens is usually dated to the archonship of the younger Pesistratos, in 522/521 BC.

There was some variation as to which deities were included, but the canonical twelve as commonly portrayed in art and poetry were :
Zeus     Poseidon    Hades   Hestia    Hera    Ares    Athena
Apollo     Aphrodite    Hermes      Artemis     Hephaestus

Source: http://en.wikipedia.org/wiki/Twelve_Olympians