Earth is the planet in which humans and the divergent mutants live. It is inhabited by those such as Spider-Man, the X-Men, Iron Man, the Fantastic Four, Hulk, and the Avengers. Galactus tried to destroy it. However the planet reminded his herald, the Silver Surfer, of Zenn-La and stopped the giant's feeding causing the two to go their separate ways.
There is another Earth called Counter-Earth in a parallel dimension. There are several other alternate dimensions.