Earth is a rocky planet orbiting the Sun, and the third planet in order from the star it orbits. It is the home of the human species, and the Turtles.
The Kraang infiltrated the Earth centuries ago in order to terraform the planet to its specifications. They have even contributed to the evolution of mankind. The Triceratons even destroyed the Earth in order to get rid of them, but the Turtles, April O'Neil, and Casey Jones went back in time to stop them.