As a Zone bounty hunter in S.T.A.L.K.E.R., you won't be alone in scavenging the irradiated land in search of artefacts and selling them off. Yep, like-minded bounty hunters are out to make cash too but in addition the Zone plays host to other factions that are operating in the area. In the developer diary below, GSC Game World talks about in-game factions in detail...
Stalkers: Stalkers are the Zone bounty hunters. They are daring souls that venture inside the Zone facing severe radiation, the anomalies as well as aggressive mutants and numerous other threats, to scavenge the area in search of artifacts. Stalkers are effectively marauders as they sell the artifacts found in the Zone to earn their living. A dangerous penny to earn and many stalkers never return from their expeditions. Those who are able to survive find their own way - either stay a lone wolf or join a community of other stalkers sharing similar values as their own. And there's plenty of warring factions in there.
Duty: Duty represents the remains of the military survivors who got stuck in the Zone. Long ago in 2009, when the first stalkers stepped into the Zone, the government decided to take the Zone territory under control. With this goal in mind they undertook a number of military operations and deployed specially-equipped forces. However, the unexpected anomalies led to many deaths and those who managed to survive became trapped in the Zone. Eventually they settled in the area presently located around Duty's bar where soldiers and officers fought with monsters, coped with hunger and deprivation, and a few of them were able to survive. When the Zone passages got less dangerous they returned to their units. The government, having such highly-trained Zone-friendly combatants on-hand, sent them back to hold the key positions, cleanse the area from mutants and pave the way for mass-scale military operations to clean the Zone from stalkers and monsters.
The leader of the grouping is a brigadier general Voronin - a stern and resolute type of person. Having undergone a personal survival test in the Zone, he enjoys respect of his subordinates.
Freedom: Freedom is a volatile mix of anarchist and terrorist. They deem the Zone as test ground for Third World War weapons and the military are eager to wrestle this from their hands and take the entire Zone under control. The goal of the organization is to prevent the military in implementing their plans, hence heir severe opposition of Duty and the army. Their activity is focused on diversions accomplished with particular brutality, fighting for free access to information about the Zone.
The leader of the grouping is Lukash, an ambitious savage. He is not the founder of the grouping however. The true leader went somewhere into the Zone center where he is rumored to currently abide.
Monolith: Stalkers of this clan were a grouping that believed an anomalous formation named Monolith exists in the heart of the Zone. It is said to contain powerful energy and is rumoured to be responsible for the emergence of the Zone itself. This is why protecting the Monolith became the primary goal of these men. Then something occurred to leave nothing human in the Monolith stalkers. They appeared unchanged visually, but their behavior became increasingly erratic. Now they fight against anyone who is unlike them - stalkers namely.
The name of their leader is unknown.
Loners: Not all the stalkers belong to groupings - there are a large number of stalkers who are independent and play their own game.