Rules Crusade is a Skirmish style wargame that handles the forces seen in 40K, Necromunda, Gorka Morka, Space Hulk, and Mordheim. RC is based on Milton Bradley's Space Crusade board game, itself a GW licensed product that was only available in Europe and Australia. The basic rules set appealed to me, and my friends and I have since expanded the rules set to support all the fantastic models and background found in other Games Workshop products. While the official 40K rules are oriented towards large, one-off battles, you'll find that Rules Crusade has more detail and thus is better for campaigns and skirmish level conflicts. Many people can't afford to field more than a squad of miniatures at a time these days!
In Battle Campaigns, such as Space Marine crusades or Eldar campaigns, several players team up, running a squad each against a common foe (similar to the old Players Vs. Gamemaster method seen in many RPGs). This format avoids the time consuming arguments and lack of moderation seen in a 1 Player vs. 1 Player environment. As a result, most of the statistics and abilities given for various forces are defined in terms of "good guys" (forces the players play) and "bad guys" (forces the GM sets up for the players to fight).
Adventure Campaigns such as Necromunda or Mordheim, on the other hand, are very competetive, but in the Rules Crusade version, any number of teams (usually at least three) are involved in a fight at a time, yielding highly entertaining and very political games. Full campaign rules exist for team creation, maintenance, promotion, skills, etc.
Ork and Chaos Campaign games start out like Necromunda, with different clans or warbands fighting each other. Eventually the clans accumulate enough to build a ship or capture a drifting Space Hulk, and begin an Off-world campaign consisting of raids and invasions in the Battle Campaign style, with the players teaming up against a common foe.
Designer: Sean Patten