The Red Forest is a woodlands area north of the Army Warehouses. In the game as well as in real life, it got its nickname from the 1986 Chernobyl disaster which killed all the trees and turned their needles a bright red colour.
In Clear Sky, the player goes to another part the Red Forest, pursuing Strelok. There is a tunnel that leads to the other part of the Red Forest, featured in Shadow of Chernobyl, which Strelok destroys so that Scar cannot apprehend him. Prior to the emission that begins the game, it was deemed impossible to get to the Red Forest, due to anomalous fields and due to the Brain Scorcher blocking the other parts. The emission opened pathways to the Red Forest, and some Stalkers ventured into the Red Forest hoping to strike gold. Among those Stalkers were a group of Renegades, Loners, Duty, and a little while later, Freedom. The Red Forest is home to the Forester. It also has a bridge that leads to Limansk, which needed to be lowered so that Clear Sky and Scar could advance through Limansk and into the Chernobyl NPP area.
Originally, in Shadow of Chernobyl, it was going to be called Radar (left-over proving this is a road-sign in Freedom HQ in Army Warehouses), and was similar to the Garbage as it had peculiar line-shaped garbage piles; and hardly any trees.
It should also be noted that in Build 1935, the now-entrance to Pripyat used to be a direct road to the NPP.
The Red Forest was known as the Wormwood Forest prior to the 1986 disaster.
In the real world, the Forest is a highly contaminated area, with over 4000 rads, compared to the outside of the NPP, which has only 350 to 42, and the median lethal dose for radiation is 300-600 rads. This is primarily because of the fact that the soil absorbed heavy amounts of radiation, unlike the NPP which was primarily laid out in concrete. A real problem are the Boars who have populated the area due to the absence of significant human population, who often attack the people who still work there, which is why military patrols are often armed with sniper rifles to repel them.
Red Forest took it's name thanks to pine tree's reddish-brown color. The discoloration is due to large amounts of radioactive fallout to which trees were subjected to and consequently died.
In Shadow of Chernobyl, when the player first comes under fire from an RPG-7u, there's a different road that goes North toward a Monolith outpost. After the outpost there's a tunnel similar to the entrance to X-19. Inside that tunnel is a door which is the same as those found inside secret labs. The door cannot be opened, and it is not mentioned in-game. In Build 1935 this was the entrance to Lab X-10, an earlier model of Lab X-19.
Originally, Shadow of Chernobyl was going to include another part of the Red Forest, in the same vicinity as the part featured in Clear Sky.
In S.T.A.L.K.E.R. Shadow of Chernobyl, a diary of a Monolithian refers to this area as the Dark Forest and Black Forest.