S.T.A.L.K.E.R. is a Ukrainian-based game that centers itself around the city of Chernobyl and Pripyat, both engulfed in the radiation of the 1986 Chernobyl reactor meltdown. The game sets you on a path to kill another STALKER, though that isn't really important in this context.

STALKER is an abbreviate term for Scavengers, Trespassers, Adventurers, Loners, Killers, Explorers, and Robbers, generally what most of the people that enter, 'The Zone' are. Framed men seeking asylum, put to work to pay off crime debts, curious onlookers who wish to wonder into the unknown world of the Chernobyl disaster area so on. All come to seek what lies in the ruins of the city and the surrounding area.

I'd explain more, but its much more fun, at least for me personally, to look it up on the STALKER wiki, as also you won't get personal preferences, opinions and such.