The Overworld is the starting location (or plane) of The Player. It is one of the three accessible worlds, the others being the Nether and The End. The Overworld is where the main character spends most of their time, simply imitating Earth. There are twelve types of biomes in the Overworld, these being the Desert, Jungle, Forest, Mushroom Island, Tundra, Taiga, Plains, Mesa, savanna, Swamp Biomes and the Extreme Hills biome. Looking at World size, The Overworld measures 60,000 km by 60,000 km (30 million blocks in each direction from world's centre), giving a surface area of 3.6 billion km2, or about 7 times the area of Earth's 510 million km2.
The Overworld is similar to Earth, except with mythical monsters such as Creepers and giant spiders. The Overworld is by far the most diverse of the three worlds. The Overworld is the only world with Water, Weather conditions, biomes, a large diversity in other plant life and animal life, and a large diversity in minerals and other materials. The Overworld is also the only world where time functions properly and mankind has made civilization. The overworld is also noted to be the only world/dimension where direction applies, whereas the Nether cannot have a compass function correctly.
"History of Mankind"
The Player was once the only human in the Overworld, until Villagers were added in Beta 1.8. However, there are multiple signs that there were once more humans across the Overworld. Notable examples are Zombies and Abandoned Mine Shafts. Seeing as Villages and AMS's were introduced at the same time, it is plausible that the shafts were built by Villagers. However, this doesn't explain Zombies, which are dressed similar to the Player. Skeletons are obviously human corpses re-animated. But how were these two mobs brought back to life in the first place? This question has yet to be answered. It is rumored that Herobrine does build most of these things, such as strongholds, but his existence is still under debate.