The Core Region is a portion of the Fallout Earth that corresponds to the twentieth century’s American West Coast and southwest in general. It was the focus of the Fallout games prior to Fallout 3 and, as such, has much more detail to it than other regions of Fallout Earth. It is the setting with which fans of the computer games are most familiar.
The basic Core Region stretches from Los Angeles in the South to Klamath in the North. It covers most of California, as well as parts of Oregon and Nevada. It is a largely inhospitable region, which is why most people have chosen to seek refuge in the remains of old cities, where they have found (relative) safety in numbers. However, as resources grew more and more scarce, people and creatures came to consolidate into groups of their own kind, so locations are far more homogeneous than one might expect from a world ravaged by mutation.
The ghoul population of the wasteland was mostly clustered around the pre-War city of Bakersfield, California (now known as Necropolis), but were scattered after an attack by the Master's army. Most of the surviving ghouls eventually found a home in Gecko or Broken Hills along with humans and super mutants, both far to the north-east of their former home.
Super mutants were plentiful in the Mariposa area from 2102 to 2162, and they sent patrols as far as to Los Angeles. After the Master's defeat, however, most of them retreated to the east.
Humans are the dominant species of the wasteland, and their main cities in the Core Region are The Hub, Los Angeles Boneyard, NCR, Vault City, New Reno and San Francisco.
There are very few factions in the Core Region that aren't limited to one or two towns. After the Master's defeat, the New California Republic became the most influential organization that unites several major towns, including Shady Sands (its capital, now known mostly simply as NCR), the Hub, Junktown, Maxson and Dayglow. The NCR is actively trying to unite all of California under a single government.
In contrast, the Brotherhood of Steel is a mysterious techno-religious organization, with roots in the US military. While few in number, they make up for this with their arsenal of pre- and post-War technology: They have laser weapons, Power Armor, surgical enhancements and combat implants. Unlike the NCR, the Brotherhood is not interested in conquest nor in saving humanity, but instead in maintaining their secrecy as well as preserving and developing technology.