Land of Ooo

The Land of Ooo is the main setting of Adventure Time, and is the home of Finn and Jake, along with all of their friends and enemies. It is divided into different kingdoms, and currently the only known kingdoms are: Ice Kingdom, Candy Kingdom, Fire Kingdom, Cloud Kingdom, and Lumpy Space, which is technically a kingdom, as it has a princess. Another non-kingdom area is the Evil Forest, and it is unknown if there is a ruler of that area.

Post-apocalyptic setting
Pendleton Ward, the creator of Adventure Time, has stated in an interview that the Land of Ooo is actually a post-apocalyptic world, the result of some sort of global disaster, hence the ruined pieces of modern technology scattered across the land. The beginning of the opening sequence briefly shows a wasteland peppered with nuclear bombs and other weapons. (There is also a scarred arm that rises from a tree at the beginning.) So far the nature of the disaster has not been revealed, and none of the characters have been shown to know about it. This also explains the origins of the frozen Businessmen in "Business Time" and the half-buried car chassis and broken TV sets seen everywhere. In "Ocean of Fear" the viewer can see a wrecked tank and aircraft carrier, as well as the remains of a city on the ocean floor, even though there are Shrimp seen swimming about. The cause may have had to do with radiation, as sometimes, mutated animals may be seen. The River of Junk also probably is all the junk left behind afterwards.