Each player chooses an army from the official list (see below) and constructs an army of pewter and plastic miniatures representing the various units in that army. Rules for constructing armies are contained within the Warhammer 40K rulebook, and army-specific "Codexes" that contain specific units and rules for each race.
Play is divided into turns; with each player choosing specific actions for all of his units on his turn, and using dice to determine the results of those actions. Each match, at the onset, is assigned a set of rules and a goal (collectively called a "scenario") specific to it. The simplest of these is a basic "cleanse" mission, which ends when either player's entire army is eliminated; more complex goals can include night fights, take-and-hold missions, and various others.
Some players organise a series of scenarios (called a campaign) where two or more players fight against each other in a number of battles. These campaigns may feature their own special rules, and are tied together by a storyline, which might alter according to the results of each scenario when it is played.
Warhammer 40K is the science fiction companion to its sibling game, Warhammer Fantasy. Warhammer 40K allows for less regimental, formation-based movement, and deals with more advanced weaponry. Other miniature-based games in the Warhammer 40K universe include Battlefleet Gothic (a spaceship wargame), Epic 40,000 (a much larger scale war simulation, with much smaller individual miniatures), Necromunda (gang fighting in a large underground slum) Gorka Morka (similar gang fighting based on a desert planet, with a focus on scavenging) and Inquisitor (an even smaller scale fight simulation, with relatively large models).
As of late 2002, Warhammer 40K is in its third edition. Fourth edition is tentatively scheduled to be coming out within the next year. New players wishing to start playing should expect to spend upwards of $200-300 US dollars for a reasonably sized army (1000-2000 points worth of models), including costs for rulebooks, and paints.
Several computer games have also been based on Warhammer 40K, such as Chaos Gate and Space Crusade.