Theogony (meaning "birth of the gods"), is a poem that was orally composed by Hesiod in roughly 700 BC, and outlines the origins and genealogy of the Greek gods. It is considered to be Hesiod's first work. Hesiod, along with Homer, is considered to be one of the greatest early Greek poets. He, with Homer, is credited by ancient authors as the establishers of ancient Greek religious customs.
Theogony begins with Hesiod sharing personal information about himself: he describes that is from Boeotia, Greece, where he works as a farmer. This is the first time that an author gives the reader (or, in his era, orator) personal information about himself or herself. After invoking the Muses for inspiration, Hesiod begins Theogony with a description of the universe before the gods — a universe of nothingness. Theogony therefore serves as a universe creation myth (cosmogony).
He then begins to describe the origins of the first goddess, Gaia (earth), and how everything in the universe spawned from her. Thus, Theogony outlines and describes the Greek gods' origins, relations to one another, struggles/conflicts amongst one another, and how they came to rule over the Cosmos. Because his work on mythology was so extensive, modern scholars refer to him in order to gain understanding of life and religion in ancient Greece.