The Dimensional Door is a kind of portal resembling an Iron Door with a blue, rift like, portal behind it. Entering the door will teleport the player to another linked Dimensional Door somewhere in the game world. Placing an iron DimDoor will create a pocket dimension, and entering that door will teleport the player to that pocket dimension. More Dimensional Doors can be placed within pocket dimensions, creating more, and this can be continued indefinitely. Each pocket created leads the player farther and farther from the light of the world they lead from.
You can also go into an already existing pocket dimension by simply putting a Dimensional Door on a Rift. Rifts can spawn naturally or will be left behind when a Dimensional Door is broken. You can also place rifts with the Rift Signature and use the door to connect them, Also, if you enter the Dimensional Door walking backwards, you will simply go through the Dimensional Door like a normal door.