The Olympian Gods are the great Pantheon of the Greeks as the Greeks worshipped during the time of Homer and later into the Classical, Hellenistic, and Roman eras. There is a generally accepted list of twelve Gods who are said to be "The Olympians" but the truth is that Olympus itself seems to be the home to a great many beings in the mythos of the Hellenes. From the Great Olympians to their children, lovers, and even some heroes. Some of these Gods can be seen as aspects of others, or even the different names of the Gods in different areas of the Greek world (Enyalos and Ares, for example, are clearly the same deity, but named differently in different areas of Greece)
Some are said to be visitors to Olympus, or underlings, such as Ganymedes, who was a mortal youth until Zeus took him as lover and cupbearer. Hebe too is cupbearer, and wife of Herakles, son of Zeus. But Olympus is not heaven. Man is not destined to dine in the halls of the Gods, man goes elsewhere, if one believes in an afterlife, which I do not. Those beings who are said to be raised to Olympus are raised so especially by the Gods.
The cults of the Olympian Gods are many, and the epithets they bear are even more numerous. But in general, the number is the seemingly significant twelve or thirteen:
Zeus: King of the Gods, father of Gods and men. God of the sky, thunder, and storms.
Hera: Queen of the Gods, goddess of marriage, goddess of oaths, goddess of retribution, and Queen of Heaven.
Poseidon: Earth Shaker, surrounder, God of the sea, God of the sea storms.
Athena: Virgin Goddess of wisdom, war, justice, victory, and mover.
Apollo: God of light, healing, medicine, music, civilization.
Artemis: Virgin Goddess of the hunt, the wild places, and childbirth.
Hephaestos: God of the forge, fire, and later associations with volcanos and industry. Also a God of metallurgical art.
Demeter: Goddess of the Earth, grain, bread, and the cultivation of the fruits of the earth.
Hermes: God of travelers, messengers, angels, and guide of the dead.
Aphrodite: Goddess of love, eroticism, beauty, flowers, laughter. Bringer of joy and good feeling.
Ares: God of war, protector of cities, god of male sexuality and eroticism, god of rage and strong masculine emotion.
Hestia: Goddess of the hearth, fire, the home, and the home cult.
Dionysos: God of the vine, wine, intoxication, initiation, insanity, and lord of the serenity and clarity of the altered state.
In addition to these, there are Gods and Goddesses who play an important part in the working of the world and are to be considered Olympian in stature if not in actuality.
Persephone: Queen of the dead, lady of the spring time, of the blossoming of spring.
Hades: Lord of the Underworld, king of the dead, the unseen, the quiet, the inevitable.
Hekate: Guide of the dead, bearer of torches, protector from maliciousness.
Helios: Lord of the Sun, the all-seeing.
Selene: Moon Goddess.
Here we finally arrive at the Gods as we know them who follow the Olympian tradition. These are the Gods who are most often thought of when one speaks of the "Greek Gods." The reason for this is actually quite simple, by the time of Homer, these Gods, especially the great Olympian Gods, were what is called the pan-Hellenic deities, the deities whose names, epithets, and iconography were known throughout all of Greece. They were not necessarily worshipped the same way all over Greece, but they were known and throughout Greece their cults in both private and as part of the city cults were almost universal.
Throughout Greece, Zeus was seen as the high God, the greatest of Gods, etc. Throughout Greece, the cult of Demeter celebrated in Eleusis with the Great Mysteries was thought of as sacred, and it is this pan-Hellenic set of deities as well as culture and language that allowed the Greeks to retain a certain unison in the face of outside influence and political and military invasions.
Today these same Gods still allow people form highly disparate nations, cultures, languages and theological backgrounds to come together and worship them, and this is what Hellenismos is.
But the mythos of the Gods gives us an indication of a fundamental truth about man's relationship with the Gods because it reflects in its stories, and especially in the theogonic tales, the conflict that this relationship tends to engender in humanity. Imagine if you will the Indo-european tribes that after centuries, and even millenia, had reached the shores of Greece. It wasn't like the Beetles invading America, it was a slow process occuring over centuries. These tribes arrive, settle, they trade and intermingle with the locals. Perhaps they were even partially assimilated by the locals. Then more tribes, also Indo-European, but a century or more later, arrive and they too intermingle, trade, and even have conflict with the locals. Some of these conflicts are religious, some economic, some political, some territorial, etc.
The Mythos reflects these challenges and conflicts between peoples, related people to be sure, at least the Indo-Europeans, but not identical anymore than a Mexican and a Spaniard are identical even though they both stem from the same cultural and linguistic background. The Olympians are not simply born to their positions, they must take them by force. The Titans do not welcome the Olympians, rather Chronos attempts to control their power, make it his own, and in doing so he does what many Kings have done in history, brought about his own doom.
The Olympian Gods, then, fight and conquer to achieve their power, their metamorphosis, and in doing so reflect for us that which our own ancestors had to do, and in the end, the Indo-Europeans who called themselves Hellenes won out, and the Greeks as we know them came to be, and with them, the great Olympian Gods had their victory, and Great Chronus was forgotten in the cultus of the Great Gods, "Οι μεγάλοι Θεοί".
Before you come to the conclusion that I am saying that all myth is just human history twisted into fantastical stories, allow me to add here that this is only part of the truth. The Gods, in my opinion, did not have a vast war for supremacy, rather they ravaged the very cosmos in their work to bring some form of order to it while at the same time balancing it with entropy and chaos. Mankind perceives the echos of this as we perceive the Gods themselves, in imperfect ways, and as a result we find the Gods at war in our myths. The evolution of their mythic and theological iconography too is reflected in the myth, and in that sense it is man's understanding of the Gods that we see reflected in myth.