World

A world is a place in the Kingdom Hearts series. Prior to Kingdom Hearts, the worlds were separated by barriers, making it impossible for different worlds to communicate with each other. The arrival of the Heartless caused the barriers between the worlds to disappear, and some worlds were even consumed by the Heartless. Gummi blocks are actually pieces of these broken barriers. There are over 25 worlds so far, 8 of which are Kingdom Hearts series exclusives, they are not part of any Disney canon. 3 of the worlds are contained within other worlds; 100 Acre Wood in Traverse Town and later Hollow Bastion, Space Paranoids in Hollow Bastion, and Timeless River in Disney Castle. Each world has it's own distinct look, and for the most part hold true to the movies and cartoons they are based off of.

In Kingdom Hearts and Kingdom Hearts II, Sora, Donald, and Goofy use the Gummi Ship to travel between worlds. As the Keyblade bearer, Sora is told not to meddle in the affairs of other worlds, however, the disruption of the Heartless and the Nobodies force him to set things right.

In Kingdom Hearts: Chain of Memories, the worlds Sora visits are recreated from his memories. He visits different worlds on each level of Castle Oblivion.

Trivia
For several worlds, its name doesn't seem to do it justice. For example, the world Halloween Town is made up of Halloween Town and, oddly enough, Christmas Town. Another more famous example is Neverland. Sora and company get to go through Captain Hook's ship and fly around Big Ben, but never actually reach the island Neverland, where Peter Pan and Tinkerbell live.

A logical explanation is that these are game levels and must be seen with a "gamer's eye."

A "Kingdom Hearts character's view" is that these names are what come's up in the Gummi Ship's navigation system.