The Overworld is the starting dimension in the game of Minecraft. Minecraft Diaries mainly takes place on the Overworld. If a Obsidian portal is created on the Overworld, it provides access to the Nether. The Overworld is where most of Minecraft Diaries, MMW, and Minecraft Diaries Side Stories takes place. (You could count FNAF and Hide and Seek as taking place here as well)
The Overworld varies between areas. It is like the real life world, in a blocky form and different in aspects. In vanilla Minecraft and modded Minecraft, it is split into biomes, like deserts, forests, oceans and beaches, hills, and grasslands as some examples.