In The Zone you will face many factions. from tourists to bandits, some friendly, some hostile. Joining a faction can make life in The Zone easier and harder at the same time, aliences, enemies and shared resources.
Loners are a nomadic semi-unified group of Stalkers who simply live off the Zone in order to earn some coin for their daily lives. Loners are neutral toward almost all other factions, but do have complications with the Military, and, to a lesser extent, the Mercenaries, and openly oppose the Bandit faction for their actions against free Stalkers.
Most Loners are only concerned with the riches of the Zone, and do not pay attention to the other factions' hatred for each other - provided, of course, they do not threaten them. Because of this, Loners are often recruited by various factions as mercenaries.
The Bandits are raiders and marauders of the Zone, ranging from petty thugs and gangsters to organized crime syndicates. Apart from Freedom, which treats them with marginal contempt, everyone hates the Bandits due to their nefarious acts of robbing and exploiting honest Stalkers. Most Bandits are unorganized and leaderless; there are only a few known Bandit leaders and camps, and even then their groups are still fairly disorganized, with underlings always trying to take control by overthrowing the previous leader. Their unit size is mediocre, and their equipment is cheap and shoddy; because of this they've adapted to a guerilla style of warfare, ambushing small groups with large numbers by camping in locations just off the normal paths.
The Mercenaries are a private military force operating within the Zone. They are professional soldiers who will do just about anything in the Zone, ranging from retrieval, assassinations, body-guarding, etc. Though, of course, it must be for the right price.
Mercenaries are usually neutral toward other factions, as they are a source of income, unless they interfere with their operations.
They are not a true factions since most are employed by different employers, so most squads serve different goals than others.
Freedom is a clan of stalkers who fight for free access to the Zone, and their views are somewhat anarchistic, believing nobody can own the Zone and seeing it both as a scientific marvel and as a free life inside the Zone's borders. In contrast to the regimented, highly disciplined atmosphere of Duty, the members of Freedom have a much more relaxed, informal attitude. They appear to have a more casual command structure, often referring to each other as "bro", with a number of members shown making regular recreational use of alcohol and marijuana. This further reflects their status as a clan of like-minded free spirits, unlike the ex-military soldiers of Duty.
Duty was founded by former military expedition survivors and Stalkers concerned about the looming horrors of the Zone. They are one of the largest Stalker clans in the Zone. They have a strict military code and operate in a military fashion, using ranks like Sergeant and General. Their goal is to contain and destroy the Zone, fearing that it will spread to the outside world. They view the Zone as an ulcer in the world and their efforts are a way to excise it. They do this with blunt force by killing mutants, eliminating all who oppose them or exploit the free, and burning down or drowning mutant dens. They are neutral toward other Zone factions, save for the Bandits and their long-time nemesis: the Freedom faction. It is said that Duty also has deep connections within the military, specifically Spetsnaz detachments within the Zone, although they treat regular grunts with utter contempt.
Clear Sky is the most secretive faction within the Zone. The faction was disbanded after losing the majority of their manpower during the events in STALKER: Clear Sky.
The Clear Sky faction was originally composed of former researchers of the C-Consciousness who broke away from them in order to conduct their own research on how to either destroy or contain the zone.
The other members of the faction were ordinary stalkers who joined their cause for their own reasons.
Monolith are a religious cult that is hostile to all but their own, the Monolithians believe in the power of the Wish Granter, rumored to be an alien artifact hidden in the center of the Zone. Members of this faction devote their lives to protecting the secret of the Great Crystal, ensuring that no one ever reaches the center of the Zone. A typical Monolithian is a former stalker brainwashed by one of the psychic emitters throughout the Zone (namely the Brain Scorcher) into serving the will of the C-Consciousness, unflinchingly loyal and ready to sacrifice himself at a moment's notice. This extreme loyalty is achieved by creating intense religious fanaticism in the mind of the stalker, directed towards the Monolith.
The Ecologists are Ukrainian scientists sent into the Zone on behalf of the government, but seem to be affiliated with outside organizations. Their main purpose is the study of all aspects of the Zone, using scientific methods to further explain what the Zone is, and its purpose. The Ecologists are neutral to all factions, and are not involved in any of their conflicts. Due to the military's slipping grip on the Zone, the Ecologists rely on opportunistic Dutyers, Loners, and Mercenaries to provide them protection, and to perform errands that may require combat, since the scientists are better thinkers than fighters.