As a player you are free to explore the entire world of the Zone. No physical restrictions. No time limits. You will become a Stalker - a cross between a mercenary and treasure seeker, who is also seen as a plunderer, a scavenger, a loner and, as food.
Your task is to locate and retrieve artifacts from anywhere within the Zone.
It's a bit like picking red hot embers out of an infernal oven. And that's the easy part.
Once you've found a few artifacts, you have to find a dealer who you can trade with.
A) They're not on the High Street.
B) They don't belong to any bona fide business association.
C) They'd just as soon cut your throat as cut a deal.
Assuming you can negotiate with them, you'll walk away with some cash.
Now you can buy weapons, ammo, protective suits and other essential stuff which will enable you to complete more onerous tasks in more inaccessible areas.
Though you work mostly alone, you can, through your behaviour, gain mutual benefits from other Stalkers. You can meet and exchange information or trade goods. But, behave like a badass and retribution will be swift.
All your actions have consequences and will impact on the world around you. Not only that, this world exists whether you are there or not. Time passes, the weather is notoriously changeable, anomalies assault the landscape, players and characters interact and feed off each other.
Things are never as you left them, or, how they seem.
The longer you survive, the more you learn about the ways of the Zone. You develop your skills and improve your reputation, you also find out how not to die of starvation. With luck and intelligence you may gain the respect of other Stalkers.
However, that still leaves the military, the mutants and the unfathomable upheavals of the blowouts to contend with.
Good luck. Or do we mean Goodbye?
Source: The Game Website.
Note: S.T.A.L.K.E.R. stands for "Scavenger, Trespasser, Adventurer, Loner, Killer, Explorer, Robber."