Earth, or the "Dragon World" is a planet based on real-life Earth, but differs is many ways. This Earth is the main setting of manga written by Akira Toriyama, including Dragon Ball, Dr. Slump and Neko Majin.
Earth (地球 Chikyuu), also called the Dragon World, is the main setting within the Dragon Ball Universe. It is inhabited by Humans, Demons, Vampires, and aliens including Saiyans and Namekians. It's similar to the more accurate depictions of Planet Earth in anime and manga in several ways. This Earth has one moon, orbits one sun and seemingly has the same neighbouring planets (Jupiter appears in Dragon Ball Z). It also has a similar land mass-to-ocean ratio. However, technology on this planet is much more advanced than modern day Earth, and the continents are different. It also has a completely different history. This Earth is similar to the Pokémon World in that respect.
In Dragon Ball, Goku is sent to Earth in an Attack Ball as a baby to conquer it so it can be sold by the Planet Trade Organisation run by Frieza and his father. However, shortly after arriving on Earth, baby Goku hits his head and forgets his mission and is adopted by an old man named Gohan instead.