The Nether
The Nether is an alternate dimension of lava, fire, and a bunch of pesky monsters. It can be accessed by creating a Nether portal in the overworld. A Nether portal can be made by getting 10 to 14 blocks of obsidian and placing them in a 4 by 5 rectangle frame and lighting it with a flint and steel.
Terrain
The Nether is essentially a giant cavern/cave in the center of the "planet". It is composed mostly of a block called netherrack. There is also a high amount of lava pools and falls. On the ceiling there is a vast quantity of glowstone veins, which act as lighting. There is also a bunch of blocks called soul sand, which slows you down when walking on it, and nether quartz, which is the only ore in the Nether. There are also structures, called Nether fortresses, which contain special blocks and monsters.