The world of Dragonball Z is similar to the real world, in that it has a planet Earth with the respective planets like Mars and Saturn orbiting the sun. However, there are some noteable differences.
Strength in Dragonball Z is often measured by power levels, or energy levels. Humans typically have a low power level, ranging from 1(an infant) to about 10(world martial arts champion). Those humans whom have mastered their inner strength, better known as chi or ki energy surpass even this, gaining a power level range of about 50. These humans are capable not only of hitting incredibly hard, but also of flight and shooting energy attacks. Humans are not the only people living in space though. The last of the Saiyans, hailing from the destroyed planet Vegeta, have made themselves known. A natural warrior race, their strength increases with every fight. As their home world was destroyed by the terrible tyrant Frieza, there are few of them left. Currently, there are only two full blooded Saiyans that are listed as being alive: Kakarot and the Prince of Saiyans, Vegeta.
One of the other prime concepts in this theme are the dragonballs. Seven golden balls, which when brought together, grant a wish. These wishes can be anything, whether it be for a truckload of chocolate, or to bring someone back from the dead. The trick is finding all seven balls, as they're scattered across the Earth, lying in hard to reach places. After the dragonballs have been used, they scatter across the Earth once more, and are not able to be used until a full year has passed.
This theme takes place just after planet Namek was destroyed. Due to the mystical dragonballs that were also outright obliterated when the planet exploded, it caused a mystical shift in the Universe. This shift cause the DBZ verse to shift and merge with the Multiverse. Earth in the DBZ realm has now molded with the other Earths, as has the entire galaxy. Frieza has been defeated, and is presumed dead. Goku is hurtling through space, his spaceship taking him to an unknown destination.