Geiger Counter is a cooperative survival-horror story game (that is, a rules-light RPG with the emphasis on collaborative story creation rather than on improvisational roleplay or min-max play). Its focus is on creating roleplaying sessions that have the feel of a survival horror movie; the goal is for all but one or a handful of the players to die, in some grisly fashion or another, at the hands (or claws or tentacles of whatnot) of some horrifying menace.
The specifics of this are, however, left to the players to devise. As is increasingly common in story games and indie RPGs, there is no gamemaster; since Geiger Counter's theme is cinematic, instead each 'scene' has a 'director' who decides on setting, plays the menace for the duration of the scene, and says "Cut" when he thinks it it time for the scene to end. Everything else within the scene is up to the players -- potentially including the director, if his own character in the game is present in the scene. Scenes are supposed to be short in duration, in keeping with this cinematic style, which typically features short, rapid scene cuts for tension-increasing purposes.
After agreeing on elements of the setting and menace, and brainstorming essential aspects of their characters, players improvise "the movie's trailer," an interesting and useful technique, ensuring that everyone agrees on the gist of the movie, and devising some interesting moments that they may try to fit into the game subsequently. (This is not essential, however; trailers can be misleading, after all.)
One player, typically the one most familiar with the game, improvises a prelude, a non-interactive scene to start things off. And then the game proceeds into a series of scenes.
A simple dice system is used in confrontations to determine who "wins;" the menace gains additional dice (up to 8) in the early stages of the game, but the players may then whittle it down in later stages until they (or the survivors) may triumph. Each time a character loses a confrontation, he or she gains a "condition," such as "lost," "injured," or "hysterical." While conditions can be reversed, any character who, over the course of the game, receives a third condition, becomes "dead." The player of a dead character continues to play, in that he or she may continue to act as scene directors, roleplay minor characters, and make suggestions.
Characters have some basic traits, and some conflicting goals, but depth of characterization is not a goal here -- any more than it is in survival horror movies, which are largely about shock and menace.
The rules provide two examples of the game in play, to give a sense of its capabilities. One is a scenario in which a scientific space station that is falling into the sun is infested with aliens; the other deals with a mysterious menace in Inner Mongolia during the Cultural Revolution, with the characters divided between Red Army fanatics and local Mongols whose culture the Red Army players are trying to suppress. The game's site also provides several examples from other playings of the game.
Geiger Counter is a pretty good introduction to the story game genre: simple, straightforward, based on a familiar trope, and intelligently evocative of its theme. And at 32 pages, it's a quick read.