There are different planets and galaxies, but they are unknown to us until some one explores it. However, our galaxy has been mapped out.
Earth: Where you start at and also one of the planet holding dragonballs.
Namek:Also one of the planets with dragonballs. Takes 6 days to get to from earth.
King Kia's planet: Has a gravity level of 10xearths normal gravity. you have to have at least a 10,000 pl. Takes 8 days to get there from earth.
Planet Freiza: Has a gravity level of 700xearths normal gravity. You have to be at least 50,000,000 pl to step on this planet. Takes 10 days to get to from earth.
Guardians: Earth and Namek have guardians, and thus have dragonballs. Any Namekian who wishes can apply for guardian of Planets without Dragonballs, then create them.
Exploration: The universe awaits, one can discover new planets outside of our galaxy. If you wish to search for new planets, you must own an exploratory spacecraft. You will receive notice on the page when a new planet has been sighted, in which someone must fly to it in an exploratory spacecraft within 6 days or it will disappear and become unavailable.