Factions:
(http://cop.stalker-game.com/img/content/cop_pers_neutral_list.jpg)
Loners:
The Loners are made up of Rookie or Free STALKERs, who are not members of any faction. Many work on their own accord, although some may do jobs for various traders or minor jobs for various factions. Equipment varies on what they are able to buy, loot or steal. Widespread throughout the whole zone. Neutral to all except bandits.
(http://cop.stalker-game.com/img/content/cop_pers_bandit_list.jpg)
Bandits:
A group of very hostile STALKERs, they share a common goal: Loot, kill, and steal from any other non-bandit STALKER. The Bandits are a nuisance to every faction, and they are the bitter Nemesis to the Loners Faction.
(http://cop.stalker-game.com/img/content/cop_pers_duty_list.jpg)
Duty:
A group of dedicated soldiers and soldier wannabes which are under command structure of a military hierarchy. Being the most organized and fundamental group in the Zone, they are trying to prevent the Zone from spreading, and waging a "crusader" like war against mutants and other factions that have bad intentions. They also train, accomplish day and night watch duties, patrol their territory, and they also do research about the Zone, mainly about how to end it.
(http://cs.stalker-game.com/img/content/cs_pers_freedom_list.jpg)
Freedom:
While Duty restricts the Zone from the outsiders, Freedom aims for the opposite; basically Freedom wishes the outer world to be allowed freely into the Zone, also using the artifacts as they wish. They also believe in studying the zone. Freedom isn't very organized; they only have a sort of duty distribution which enables everyone to do what they are good at. Known for guerilla tactics and one of the only factions to use Sniper Rifles, such as the SVD or SVU.
(http://cs.stalker-game.com/img/content/cs_pers_mercenary_list.jpg)
Mercenaries:
An elite armed forces hired by an unknown customer, rumoured to be the grunts of the other Western European countries, which are trying to understand the happenings in the Zone. Others say this fraction is a secret Russian army group, disguised in a Western-based equipment, and trying to stop the independent research and military development. Faction standings vary greatly on who the Mercenaries are, as there were quite a few groups of neutral, even extremely popular mercenaries throughout the zone.
Monolith:
A mysterious and unknown faction of Stalkers. From PDA records and logs, their intent would be to protect the NPP and the Wish Granter, which they revere. No one knows who they are aligned with, their mood or other intentions they have. They simply ward off those who stray too close to the Centre, shooting everyone on sight and never asking questions. There is no contact with them on a personal level, which leaves much speculation to be solved about them. They may be agents of the C-Consciousness. It is not quite clear how they recruit.
Military:
The Ukrainian Army which are keeping the Zone unreachable from the outsiders. They are mostly drafted soldiers with limited skills, but also have a special task force in their ranks. The Army power in the Zone is very limited, along with their equipment, because of the economic reasons and having most of their forces fighting at the barrier. However, there have been known Military expeditions into the Zone, commonly run by the very dangerous Spetznaz.
(http://cop.stalker-game.com/img/content/cop_pers_military_list.jpg)
Military Stalkers:
Another secret organization which hires stalkers to accompany with the officials, for stopping the stalker activity. They are heavily armed, mostly using silenced marksman rifles and are sent to some critical areas by the command of the Army.
Ecologists:
Operating under orders from the Ukrainian government, the Ecologists are a group focusing on researching the Zone, in order to properly harness its potential for the good of mankind. The Ecologists focus primarily on research of new energy sources, antigravity, genome modification and teleportation, spearheading revolutionary research by studying and dissecting artifacts and mutants. The Ecologists field NATO weapons when they are forced into a combat role, and their suits are more for Anomaly Diving instead of Bullet Absorbing.