According to Greek mythology, Medusa was a terrible monster who was once a beautiful maiden whose hair was her chief glory. Still, as she dared to vie with Athena, the Goddess changed her beautiful ringlets into hissing serpents. She became a cruel monster of so frightening an aspect that no living thing could behold her without being turned into stone. All around the cavern where she dwelt, the stony figures of men and animals that had chanced to glimpse her and had been petrified by the sight might be seen. Perseus, favored by Athena and Hermes, the former of whom lent him her shield and the latter his winged shoes, approached Medusa. At the same time, she slept and took care not to look directly at her, but guided by her image reflected in the bright shield he bore, he cut off her head and gave it to Athena, who fixed it in the middle of her Aegis.
From Bulfinch's Mythology
In earlier Egyptian, Sumerian, Old European, and later Celtic cultures, the snake is an enduring symbol of the Goddess; hence, the myth of Medusa may also be understood as a re-mythic of a once benign sacred symbol of the great Goddess and her snake priestesses into the horrific, and vanquished, Medusa by later patriarchal Greek mythology.