The Incipisphere is the world in which the majority of Minestuck takes place. While the vanilla Minecraft world is composed of the Overworld, the Nether, and the End, with no apparent non-magical relationship among them, the Incipisphere relates all its regions to each other physically.
As the name suggests, the Incipisphere is a sphere, though all the bodies inhabit it are found along the same two-dimensional plane.
At the heart of the Incipisphere is Skaia, where there is a constant ongoing battle between The Armies of Light and Darkness. Skaia is the body around which everything else in the Incipisphere orbits, and its atmosphere produces the light which illuminates the Medium and the Veil. In very close orbit around Skaia is Prospit.
In the further parts of the Incipisphere are The Veil, and, immediately beyond that, Derse. The Veil is an asteroid belt where some session necessities are found, such as the Frog Temple and the Ectobiology Equipment. When/if the battle on Skaia comes to an end in favor of the Black King, he will cause the asteroids in The Veil to fall toward Skaia in an attempt to destroy it. This is called the Reckoning. The smaller ones will be redirected to the Overworld through Skaia's defense portals, while the larger ones will make it through and eventually destroy Skaia if the Reckoning is not stopped.
The area between Prospit and The Veil is known as the Medium. This is where everything else in the Incipisphere can be found, including the players' Lands and their Gates. Within the heart of each Land is a Denizen, which guards an incredibly large grist cache required for Ultimate Alchemy.
Once inside the Incipisphere, it is impossible to return to the Overworld through legitimate means.
The geographical area outside the Incipisphere is known as the Furthest Ring, where the Horrorterrors and Dream Bubbles are located. In the Furthest Ring, the rules which govern reality and physics begin to break down, as Aspects begin to simultaneously blend and separate, creating an unstable reality inimical to Minecraftian life.
*Note that the information above largely relates to the Incipisphere in Homestuck, and the one in Minestuck may have certain variations from these rules.