The Overworld is the first and main dimension in minecraft. It’s the place you start out in and in which you’ll probably spend most of your time. The Overworld is the most detailed dimension and contains various biomes, mobs and blocks, most of which are only found in the Overworld dimension.
There are various biomes, most of them just look different, though some have unique blocks, mobs or other aspects. For example, it never rains in desert biomes, mooshrooms can only be found in mushroom biomes and jungle trees can only be found in jungle biomes. Besides biomes, the world is also made up out of various land marks, like rivers and lava pools. These landmarks are part of the biomes, but are generated seperatly and can sometimes be part of multiple biomes.
Almost every mob in minecraft can be found in the Overworld, though there are 8 mobs which either never spawn in the Overworld, or they require some form of input, like taming. Those mobs are the Ender dragon, snow golems, blazes, ghasts, magma cubes, zombie pigmen, cats and dogs.
The Overworld is (almost) only limited in height, which is 256 blocks high, though land never reaches anywhere near that high. There is no phyiscal boundary in the air, but you will be unable to place blocks above the height limit, unless you use mobs.
The bottom of the Overworld is covered in bedrock, which is an undestroyable block and acts as a boundary. It’s possible to break these blocks if you use mods, in which case you’ll gain access to the void. The void is nothing more than pure darkness and going too deep into it will kill you. Any blocks falling into the void will also be destroyed.
The Overworld is almost infinitely large, though there are limits. However, these limits are enormous and are never reached unless a player spends time getting to the edge, which can be found at X and Y coordinates of 30,000,000.
The Overworld is currently the only dimension with a day/night cycle, which lasts 20 minutes from one sunrise to the next. This cycle changes the light levels and affects the way mobs spawn and react to the player.
Each world has it’s own seed, which is a number used to generate the world. Seeds can be used to create a specific world as you can enter a seed before creating a new world save. A seed will always create the same world, which is often used by players to share amazing, naturally generated worlds or worlds that spawn you close to a stronghold, dungeon or other advantage. Seeds will be displayed in the information you see when you press F3, in the top left corner.