This is a guide to some organizations and things travelers in the wastes may come across. Many of these are staples of the post-nuclear environment.
The Brotherhood of Steel
A techno-religious military order estabilished by US soldiers shortly after the Great War, the Brotherhood of Steel dedicates itself to preserving humanity's progress - not by saving individual communities, but by hoarding and preserving technology. For decades, they were the sole bastion of advanced technology in the wasteland, however, with the emergence of the NCR and the Shi, they had been steadily losing their advantage, becoming a shadow of their former selves.
There are many different ranks within the Brotherhood, and three separate castes: knights, paladins and scribes. Unlike the chivalrous knights of old, members of the Brotherhood are not interested in justice for the obviously weaker and less fortunate around them, but instead in keeping their secrecy and preserving and developing technology. Their motives are often unclear, and Brotherhood members are not people to be trifled with. It is safe to say, however, that if a group of Brotherhood knights appears to be helping some less fortunate people, their motives are not altruistic. If you encounter one in the wastes, they are most likely on some kind of mission.
There are numerous chapters of the Brotherhood of Steel, from the sprawling empire of the Midwest Chapter, to the Xenophobic and Nazi-esque East Coast Brotherhood to the humble, secluded, isolationist Brotherhood of the Mojave. While the Brotherhood originated on the West Coast just before the war, they travelled as far as West Virginia within the first 25 years of the foundation. While on the retreat across America in face of strong opponents, the Brotherhood of Steel is still a dangerous faction.
Comprised mostly of reconditioned tribals and their offspring, Caesar’s Legion is a huge slaving organization that operates east of the Grand Canyon. Led by the charismatic and cunning “Caesar” (a former Follower of the Apocalypse), the legionnaires are a well-organized fighting force and absolutely merciless in their slavery. Mimicking the ancient traditions of the Roman Empire, the legionnaires dress in segmented football pad armour and decorated football helmets.
The Legion sprawls across the American southwest, stretching from the Colorado River to Denver and Arizona. While not technically a nation, Legion troops garrison many towns, from which they extract resources and weaponry. Ceaser aims to conquer New Vegas and turn the Legion into a proper portrayal of the Roman Empire. Until then, Ceaser's Legion is simply a highly efficient wandering army.
Even more technologically advanced and secretive than the Brotherhood of Steel, The Enclave is what remains of the Pre-War United States Government and Military. Originally, they were headquartered on an oil platform located off the Californian coast and use helicopter-like craft called "Vertibirds" to travel to the mainland. Their motives are to reinstate the United States and rid the planet of every mutant lifeform (including the human survivors of the war). While their ultimate goal is to rebuild the United States, the Enclave achieved this through brutality and genocide. Ultimately, this caused the destruction of the Enclave by the Chosen One in 2242.
Faced with the destruction of much of their chain of command and hierarchy, the Enclave was all but eradicated on the West Coast. Small groups of Remnants exist, but they are highly secretive about their past and keep their weapons and armour locked away. At most, a person is lucky to find one squad scattered across a whole desert.
The Enclave on the East Coast went a similar way 35 years later. Taking survivors from the destruction of the Oil Rig, a Colonel Autumn brought a large number of Enclave personal to Raven Rock, in the Capital Wasteland. Here, they again fought the Brotherhood of Steel, before being wiped out by the Lone Wanderer in 2277. This destruction is even more complete - the few remaining survivors were scattered as far north as Maine.
The Followers of the Apocalypse
The Followers of the Apocalypse are a quasi-religious group based in the ruins of the main branch of the Los Angeles Public Library in the western segment of the Boneyard. Led by Nicole, the Followers are dedicated to scholarship and to ensuring that humanity does not repeat the mistakes that led to the Great War. After the destruction of the Cathedral and the death of the Master, the Followers of the Apocalypse became a major influence first in Boneyard, and later in the newly-founded New California Republic.
Over time, however, this influenced faded. The NCR viewed the Followers as against the state. While there's no persecution of Followers, they suffer from lack of support and, on occasion, antagonisation by the Two-Headed Bear. In 2281, the Followers remain a kind charity organisation for the downtrodden, even running a university back in the Boneyard.
The New California Republic
The Republic, or NCR as it is widely known, is a strange mix of pre-War democracy and post-War dictatorship. Begun in the town of Shady Sands, now the capital (although the city itself is often referred to as the NCR), the Republic covers most of Southern California and some settlements in far west Nevada. Cities and towns send delegates to Shady Sands, which boasts an impressive post-War capitol building, and a president is elected from among the delegates. So far, all of the presidents have come from Shady Sands, the city with the greatest number of delegates, as it is the most populous. To become a citizen of the NCR, a person must agree to follow their rules, most of which are enforced by the huge NCR police force. Mutants and ghouls are not permitted the same membership privileges as humans. Slavery is forbidden in the NCR, and members of the slave guild are thrown in jail on sight. In addition, the government controls what weapons the citizens have access to, and it is illegal to brandish weapons inside city limits. Gambling and prostitution are also illegal in the NCR. The Republic grants a city membership - and, by extension, the presence of the NCR police and protection - in exchange for tax profits and control over trade. The NCR has been known to bully cities into joining, especially when that city trades in something that the NCR needs, although such bullying is usually done with embargos, not guns.
New California Rangers
Technically part of the New California Republic military, the NCR Rangers are practically a force unto themselves. The Rangers are given exceptional leeway and flexibility to deal with both the intrusion of organized crime and the operation of slavers near the borders of NCR territory. NCR Rangers typically have bounties on their head, even in NCR states.
Though New Reno is the obvious example, some cities in the wasteland are controlled by crime "families." They are generally modeled on Mafia and Yakuza syndicates of the pre-war world and deal in the economies of vice (guns, gambling, alcohol, drugs, prostitution, slavery) and have been found to work in consort with many disparate organizations to achieve their ends. Hitmen act as their enforcers and diplomats and are typically beholden to a strong sense of honor. Violent rivalries between syndicates is not uncommon.
Most towns maintain their own police forces, usually headed by an elected Sheriff, to enforce their laws. Larger groups use organized police militia forces, who often are above the law in their enforcement of it. Due process and fair trials are things of the past; more often than not, justice is fast and deadly, or at least painful. Smaller town police tend to be more willing to listen to both sides of an argument, and will be more likely to throw a criminal out of town rather than jail or execute them, although they often listen to the desires of the townspeople regarding these matters. Members of larger police forces tend to be more corrupt, and can often be bribed. Be warned that bribing police officials can result in greater penalties if unsuccessful. A group known as the Regulators function as the keeper of justice in the Capital Wasteland. However, they follow the tenants of the wasteland. This means they do not arrest evil-doers, they just kill them.
Raiders are tribes of people that steal what they need from other communities. They usually do not have villages, but semi-permanent encampments, often in the ruins of small towns. Children and older members of the community will engage in some small-scale agriculture, but Raiders mostly survive on what they can steal from villages, merchant caravans, and other Raiders. Usually, they prefer to strike and fade quickly, not so much interested in killing but immobilizing and taking what they want before the guards come. Generally considered a nuisance in most areas, but larger "kingdoms" may have sent police after Raiders in their area, eliminating the threat to trade and profit. Very rarely, Raiders may organize into larger groups. However, this is usually only for the sake of trading between different Raider parties or organizing against a very serious threat to all the Raiders in an area. This is usually a temporary arrangement Raiders in the DC Wasteland are known to be far more brutal, using terror tactics modeled after people such as Ghengis Khan and Vlad the Impaler.
The Ruling Families of the New Vegas Strip
Three organizations formed by the enigmatic ancient mogul known as Mr. House from the ranks of tribals he recruited to his cause. Like the Legion, they were reconditioned to abandon their old tribal customs, but the models for their new societies were organizations and subcultures that frequented the pre-War Las Vegas, which Mr. House personally remembered.
The Chairmen family (formerly the Boot Riders tribe) adopted the "cool" aesthetic and the Rat Packer culture. They are the most lawful, honorable and reliable of the Ruling Families, especially after their recent change of leadership; the former leader of the family, one cunning and treacherous man named Benny, was killed or gone missing, depending on which source you trust. Their base of operations is a casino called the Tops, known as the center of entertainment of the whole city.
The Omerta family (formerly the Slither Kin tribe) adopted the customs and culture of the Italian mafia of old, and estabilished their reputation as a secretive gang of racketeers. They are untrustworthy and backstabbing, but they offer services no one else can provide, such as sex and drugs trade. Their base of operations is the Gomorrah casino and brothel, infamous as the nexus of sin and vice of all New Vegas.
The White Glove Society (formerly the Sawneys tribe) is the richest of the three families, and thus they adopted the culture of the pre-War upper classes. Their reputation is fairly sinister, partly due to their snobism and putting on airs of mysteriousness, partly based on rumors of their cannibalistic habits. They were, indeed, cannibals during their tribal days, but this custom of theirs was strictly banned by Mr. House. The rumors, however, claim that not all of them abandoned the old ways. Their base of operations is the Ultra-Luxe casino, which also houses the famous Gourmand restaurant.
Unique among survivors of the war, the Shi are descended from Chinese submarine sailors that were safely submerged when the bombs fell. They now repopulate a portion of New San Francisco in an area dubbed "Chinatown." Ruled by a supercomputer they call "The Emperor," their level of technological advancement is quite high, though they lag behind the Brotherhood of Steel in many areas.
The Slave Guild
Slavery is a lucrative business in the post-War world. Although many larger pockets of civilization have outlawed slavery, smaller dictators allow the purchase and sale of other human beings. Slavery also happens to be one of the most controlled businesses in the post-nuclear world. Most slave trade in the area of California is done through the Slave Guild, a collection of older slavers who decide prices and organize capture hunts. Anyone caught capturing or selling slaves without the blessing of the slave guild is usually executed. Since the Guild holds so much power, it often controls smaller towns were it operates, although from behind the scenes. Members of the Slave Guild have a distinctive tattoo covering most of their face, making them readily identifiable to both friend and foe, allowing the guild to make fast judgments when someone without "the tat" has a tribal up on the auction block. Other Slave Guilds exist in other regions of the Wasteland, and not all mandate an identifying mark.
Where Raiders are more nomadic, some groups have settled into a simple, agricultural lifestyle, forming small villages. Some of these are little more than tents, others are large communities in ruins of towns. "Tribals" often have sophisticated - and unique - belief systems, and no two tribes are going to be alike. They have managed to find ways to make artifacts from before the War useful. Tribals are not necessarily interested in rebuilding civilization, which many view as the cause of the destruction (if, indeed, they remember pre-War civilization at all), and are wary of larger groups of people and cities. In turn, city dwellers view Tribals as primitive savages.
Before the War, a company called Vault-Tec built a series of enormous underground facilities for the US Government designed to keep people safe from disaster. They financed this project by selling places in the Vaults to people who could afford them. Although they never expected to use the Vaults, many people managed to find their way into them before the bombs hit. Vaults are three-story underground structures deep inside of mountains, where they are shielded from the effects of radiation, disease, and other catastrophes. They were designed to run for as long as necessary to keep people alive, using hydroponics technology and water-recycling. Many stayed sealed for 80 or 100 years. Vaults were equipped with sensor devices to monitor outside conditions, and had computers with vast amounts of pre-War data. In addition, they were well stocked with weapons and other important tools. Some Vaults were equipped with an item called the G.E.C.K., the Garden of Eden Creation Kit, a device that was designed to help the inhabitants build a new life once it was safe to return aboveground. Some Vaults were destroyed by earthquakes, Raiders, or other disasters, but a few of them managed to survive, technology - and people - intact. The true purpose behind the Vaults was to determine whether man could survive for long periods in enclosed places often at the expense of the unknowing Vault dwellers; the hope was to repopulate another planet via space travel and the G.E.C.K. Vault-Tec is not the only source of long-term surival shelters, so other similar facilities can exist.
The Great Khans
Modeled after the culture and traditions of the Mongol hordes, the Great Khans are one of the only truly organized raiders groups in the wasteland. Their history began in the fabled Vault 15. After the Vault was abandoned, the Khans that emerged were nothing more than simple raiders. They were eventually beaten down, but that wasn't the end of the Khans. They would resurface, time and time again only to be beaten back each time. But the Khans would meet their finest hour during the massacre of Bitter Springs. A simple miscommunication led to the massacre of men, women, and children in the village of Bitter Springs. This would lead to the Khans exile to Red Rock, and their alliance with Caesar.