Overworld
The Overworld is the dimension in which all players begin their Minecraft world. It is the dimension with the most biomes, blocks, and mobs, and is where most players spend the majority of their time. The Overworld is an incredibly complex environment with a wide variety of features.
The Nether
The Nether is a dangerous, hell-like dimension containing fire, lava, fungal vegetation, many hostile mobs, and exclusive structures and biomes. The Nether features unique cave-like terrain with no sky. Lava and fire are common hazards everywhere in the Nether. Areas distant from the player are shrouded in fog of red or blue, depending on the biome. The shape of the terrain in the Nether is independent of the biome distribution; instead, the biome alters the surface material and adds terrain features to the netherrack terrain.
The End
The End is a dark, space-like dimension consisting of separate islands in the void, made out of end stone. It is inhabited by endermen and shulkers. The ender dragon also spawns when the player first arrives. The End is the final goal of Minecraft. The ender dragon is considered the final boss, and it is necessary to defeat it to "beat" the game and see the end poem.