Origin
In the beginning, Hesiod tells us, there is Chaos. Then comes Gaia, the broad breasted Earth, who alone gives birth to the Sky, Ouranos, to the mountains and to the sea. The whole family of the gods descends from her.
The story
Gaia is a protective and formidable mother. When Ouranos imprisons their children, she arms Cronus to overthrow him. When Cronus in turn devours his children, she helps Rhea save Zeus. Every time a ruler oppresses her children, Gaia prepares his fall.
Before it belonged to Apollo, the oracle of Delphi was hers: people came to consult the Earth herself. The Romans worshipped her as Tellus, the nurturing Earth.
The symbol
The Earth is the only planet whose name does not come from Greek or Roman mythology: it comes from an old word for the ground. It is also the only one, as far as we know, that harbours life.
In astrology, the Earth does not appear in your chart, and for good reason: this is where we look at the sky from. Your chart is a snapshot of the sky taken from your place of birth.