The Nether dimension exists in Minecraft worlds. It was first seen in the Alpha version 1.2.0. It is mostly used for traveling large distances. What makes The Nether unique is it contains Block types that can’t be found anywhere else. The Nether is the home of mobs Zombie Pigmen, Blazes, Magma Cubes and Magma Cubes. 3 of these mobs are only found in The Nether Dimension. The only one found elsewhere is Zombie Pigmen which can be seen in the overworld when a pig spawn is hit by lightning.
The Nether acts as a 2nd map in your Minecraft World. The chunks from the above ground map are unloaded and The Nether chunks begin to load. If you die in the Nether your items stay there until you respawn in the overworld.