Earth is a terrestrial planet that holds the Land of Ooo, the primary setting for Adventure Time. According to series creator Pendleton Ward, the series takes place on the planet after it had gone through a series of apocalyptic events. This includes the Great Mushroom War and an unknown occurrence which resulted in a large, continent-sized crater in the Earth's surface.
Earth is first seen in "The Real You" but the first time the Earth is referenced within the series is in "Holly Jolly Secrets Part II" when Simon Petrikov (Ice King) mentions that he obtained his crown in Scandinavia. In "Five Short Graybles," a holographic visual of Earth is seen rotating (see image in sidebar) much like a desk globe. The Earth is shown again In "The Lich" when the Enchiridion opens a map which Jake enhances to show the solar system. In "Burning Low" the Earth is again shown when Princess Bubblegum conjures up a green holographic screen. The Earth is shown with its crater on this screen.
At some point a large portion of the Earth became missing. Although how this occurred is unknown, in "I Remember You" and "Betty" the Ice King is shown to have a drawing of an intact Earth with a giant meteor heading towards it. However, this may also be referring to the asteroid in the episode "Wizard."
A theory for the creation of the crater could also have referance to the Great Mushroom War. It's possible that the bomb, the one which created the Lich had such a huge explosive force that it actually moved the Earth out of orbit, causing a comet to crash into it. This evidence can be supported as in the episode Finn the Human, when Farmworld Marceline was explaining that the death of Simon Petrikov made the crown freeze the Earth for 400 years, a Blue comet could be seen passing by the Earth. So, it can be concluded that with the detonation of the bomb, the explosion caused the Earth to move into the way of the comet, causing a collision that ended in a large crater. In the episode 'The Vault', Finn views one of his past lives as a comet. Incidentally, the comet Finn viewed himself as looked exactly alike as the one which presumably collided with the Earth. In the episode 'The Comet' however, it is revealed that as a comet, Finn crashed into the Earth and became a butterfly. Thus, it could be concluded that it was the comet, or Finn, who created the large crater.
Assuming the rotating globe (see image in above sidebar) depicts the Earth as it is during the show, it can be seen that there are four continents left on the planet, but only the continent that the Land of Ooo resides on is known to have been featured in any episode to date.
The Land of Ooo is the main setting of Adventure Time and is the home of Finn and Jake along with the majority of their friends and foes. It is divided into different kingdoms as well as independent regions.
Despite the apocalyptic events in the past there remains a diverse range of life on the Earth, much of which now displays signs of sentience (e.g. talking insects, animals, and plants); however, due to the war, most of the planet's current inhabitants have undergone some form of mutation due to the radiation from the Great Mushroom War including beings which were previously not living creatures. Additionally, there are sentient non-living being such as the robot BMO.
Most of the inhabitants of Ooo speak English, but other languages, such as German and Korean, are also spoken by a certain individuals and entire species.
Science and technology
It is inferred that a great amount of scientific and technological knowledge has been lost due to the Mushroom War, such as when Jake claims in "The New Frontier" that rockets have not been re-invented yet. Despite these setbacks, there have been great advances in science and technology. Examples of this include holographic displays, the ability to design and create life (e.g. the Earl of Lemongrab), genetic splicing and mutation (e.g. Princess Bubblegum creating the perfect sandwich), and an immortality elixir used to bring long dead Candy People back to life.
Merging magic with science seems to happen frequently in the show and the two have been shown as interchangeable with scientists using magic (e.g., Princess Bubblegum creating the perfect sandwich in "Five Short Graybles") and spell-casters using science (e.g., Ice King creating a magic computer virus in "A Glitch is a Glitch."