Description of factions
Lone Stalkers are most often encountered in the southern part of the Zone, but sometimes they wander into the central Bar and even congregate near the Freedom base in the north. The player won't have any problems with them unless they attack first.
Scientists are willing to buy weapons, artifacts, or body parts of killed mutants from the hero. They live in only one place—a bunker on Amber Lake. Scientists are never encountered in the wild.
Bandits are simple thugs and criminals drawn to the Zone by the lust for profit, easy artifacts, and the opportunity to trade weapons. They can be recognized not only by their black jackets and blue sweatpants, but also by their distinctive expressions. Bandits attack anyone and everyone—and everyone responds in kind.
The army not only guards the perimeter of the zone, patrolling the areas in helicopters, but also sends in airborne troops—though you won't encounter military stalkers (as well as helicopters and armored personnel carriers) until near the end of the game. Soldiers are rarely encountered in the southern and central regions. They patrol the roads, monitor the Agroprom Research Institute area, and occasionally attack areas captured by bandits.
"Duty" is a paramilitary organization whose members destroy monsters and do everything in their power to prevent artifacts from leaking outside the Zone. Duty's main base is the Bar. As paladins, they dislike bandits, but their main enemy is "Freedom." If a player joins "Freedom," the Bar's guards will open fire and prevent them from returning... as long as they live, of course.
"Freedom" is an anarchist organization opposed to the ideals of "Duty." They advocate free artefact hunting, but at the same time protect everyone else from the northern threat. They block access to the central districts to prevent Monolith stalkers from breaking through from the north. Anyone who befriends "Duty" will become an enemy of "Freedom." Anyone who befriends "Freedom" will suddenly discover that bandits have stopped attacking them—an unspoken truce exists between the criminal underworld and "Freedom."
The Mercenaries are an organization that wages war against all stalkers. It's amazing how they manage to stay alive. The Mercenaries control the Wildlands region adjacent to the Bar. Befriending them or stopping them from attacking the hero is impossible.
The Monolith is a stalker cult. Its members worship a mysterious artifact, a large monolith that supposedly grants wishes and is located somewhere in the north. The Monolith has occupied all of the northern lands—the Red Forest, the radar station, Pripyat, and the Chernobyl Nuclear Power Plant itself. A mysterious, yet no less deadly, psionic field prevents other organizations from attacking the Monolith's positions. Few people want to become zombies. Befriending the Monolith is also impossible.
Source: stopgame.ru
See also:
Shadow of Chernobyl: Army Warehouses
Stalker: Shadow of Chernobyl - Armor
Stalker: Shadow of Chernobyl - Yantar