Earth is the third planet in the sol system, in the outer arm of the Milky Way galaxy. Most noticed for it's blue appearance due to the massive bodies of water that surround it.
Planet Earth is likely the planet anyone reading this description is living on. It is also by far the most common and popular setting for anime, even if it is severely modified for the sake of fiction. The typical Earth has 7 continents and numerous countries, as it does in real life. Some series show a future, desolate wasteland Earth.