There are seven (7) worlds which the player can choose from when launching the game. Every world offers the same as each other, although world 3 is for premium members only. The choice of world does not affect gameplay in a major way, however it does smaller differences.
Worlds 1, 2, 4, 5, 6, and 7 are all worlds for standard player accounts, and while these worlds provide no benefits between them, some worlds may be chosen for different reasons. For example, World 2 is often chosen for dungeon parties as it is most commonly the most populated world. Other worlds such as 6 and 7 are often chosen for the lower population count, where more mobs may be available and lower ping may occur.
Currently, World 3 is the only world available for premium users, which offers users lower ping counts due to the very low population count. Bosses are also available much more due to this factor.