The Core Region is a section of the Wasteland or the post-apocalyptic United States after the Great War, consisting of the west coast and southwestern/midwestern area in general. It is considered to be far more vast than the Capital Wasteland, and it appears in Fallout, Fallout 2, and in Fallout: New Vegas, although the surrounding area is called the Mojave Desert or Wasteland. The area is more fleshed out than the Capital Wasteland and old Fallout fans are more familiar with this area in the Fallout Universe. The Core Region has three protagonists or player-characters, the Vault Dweller, the Chosen One, and the Courier.
The Core Region stretches across the west coast, from Oregon to California, all the way to parts of Nevada or the "four corners". It consists of barren deserts, demolished and lifeless cities, and small beaten-up towns. Settlements are more spread out than the Capital Wasteland. The Core Region borrows themes from the "Wild West", more so in Fallout: New Vegas.
Humans are still the dominant race in the Core Region, and make up a good amount of the population. However Ghouls can be found in the town Gecko and the city Necropolis or Vault 15, and Super Mutants were found all across the west, specifically the Mariposa Military Base, before the events of Fallout 3.
Organized Raider groups such as the Khans are seen in-game, along with other factions such as the New California Republic and the Brotherhood of Steel. The Enclave appears in Fallout 2, but after a major defeat in Fallout 3, it is unlikely that they will be seen again in the Core Region.
Thanks to the creation of nuclear warheads and the hostilities between the United States and China, the world is in a state of chaos and confusion even 200 years after the holocaust.