Earth is the planet inhabited by, most notably, Humans, Nartec, The Great Ones, and The Little Ones. It is the current target for the Yeerks, who wish to enslave the human race. Elfangor lived on this planet with Loren in a different dimension using the Time Matrix. The Ellimist reversed the effects of this after Elfangor had a child with Loren that he found out about after time was returned to normal. The child, (Tobias) later became an Animorph to fight for Earth. Please see Wikipedia's real life earth page.