Theogony is a poem by Hesiod describing the origins of the gods of Greek mythology.
Hesiod's Theogony is a large-scale synthesis of a vast variety of local Greek traditions concerning the gods, organized as a narrative that tells how they came to be and how they established permanent control over the cosmos. In many cultures, narratives about the cosmos and about the gods that shaped it are a way for society to reaffirm its native cultural traditions. Specifically, theogonies tend to affirm kingship as the natural embodiment of society. What makes the Theogony of Hesiod unique is that it affirms no historical royal line. Such a gesture would have cited the Theogony in one time and one place. Rather, the Theogony affirms the kingship of the god Zeus himself over all the other gods and over the whole cosmos.
Further, Hesiod appropriates to himself the authority usually reserved to sacred kingship. The poet declares that it is he, where we might have expected some king instead, upon whom the Muses have bestowed the two gifts of a scepter and an authoritative voice (Hesiod, Theogony 30-3), which are the visible signs of kingship. It is not that this gesture is meant to make Hesiod a king. Rather, the point is that the authority of kingship now belongs to the poetic voice, the voice that is declaiming the Theogony.
After the classical period, when divinely-appointed kingship is brought into Greece once more, it will come in from outside, from Macedonia and imported from the royal traditions of Persia.
Although it is often used as a sourcebook for Greek mythology, the Theogony is both more and less than that. In formal terms it is a hymn invoking Zeus and the Muses: parallel passages between it and the much shorter Homeric Hymn to the Muses make it clear that the Theogony developed out of a tradition of hymnic preludes with which ancient Greek rhapsodes would begin their performance at poetic competitions. It is necessary, therefore, to see the Theogony not as a sort of Bible of Greek mythology, but rather as kind of snapshot of a dynamic and often contradictory tradition as it happened to crystallize at one particular place and time - and to remember that the tradition kept evolving all the way up to the time of Nonnus.
From a Lecture:
1-21 Opening Hymn to the Muses
The Text of the Theogony
translated by Hugh G. Evelyn-White 
(ll. 1-25) From the Heliconian (Memory), who reigns over the hills of Eleuther, bear of union with the father, the son of Cronos, a forgetting of ills and a rest from sorrow. For nine nights did wise Zeus lie with her, entering her holy bed remote from the immortals. And when a year was passed and the seasons came round as the months waned, and many days were accomplished, she bare nine daughters, all of one mind, whose hearts are set upon song and their spirit free from care, a little way from the topmost peak of snowy Olympus. There are their bright dancing-places and beautiful homes, and beside them the , Warfare