Space Marine is the name of the second edition of Games Workshop's 6mm table-top wargame, based on the 35mm Warhammer 40K game. Using this smaller scale allows the players to use larger and nastier hardware, more troops and more varied terrain.
Space Marine has its origins in the original Adeptus Titanicus, the Titan combat game that didn't include rules for infantry and vehicles, only Titans. The original Adeptus Titanicus boxed set came with 6 plastic Warlord Titans, a whole bunch of counters and cards, and some really cool buildings that were a lot better than the thin cardboard crap GW ships these days (I still have most of mine).
Adeptus Titanicus was originally set during the days of the Horus Heresy, in fact I think it was Adeptus Titanicus that introduced this aspect of the 40K storyline. Players could either be loyalists or rebels.
Shortly after Adeptus Titanicus was released, GW released an expanded rulebook called Codex Titanicus. The Codex introduced large infantry vehicles like the Capitol Imperialis, and Titan units for the Orks (called Gargants) and Eldar. It was basically an advanced rulebook.
Space Marine introduced infantry units and vehicles into the Adeptus Titanicus game system. The boxed set came with more buildings, around 400 Space Marines and a few dozen Rhinos and Land Raiders. Like Adeptus Titanicus, it was set during the Horus Heresy; there were no aliens and players took on the roles of Imperial and Rebel forces. Future expansions published in White Dwarf added rules for Orks, Eldar, Squats, Chaos and the Imperial Guard.
In around 1994, the disorganised and overly-complex components of Adeptus Titanicus, Codex Titanicus and Space Marine were compiled, simplified and re-released under the Space Marine title. The boxed set contained one Titan, a couple of hundred marines, Orks and Eldar, and Rhinos, Battlewagons and those Eldar tanks whose name escapes me. Again, the system was expanded in White Dwarf issues with periodic releases of expansion sets for Marines and Imperial Guard, Orks and Squats, Chaos and Eldar and the Tyrannids. In 1996 Titan Legions released with additional rules for Emperor class Titans like the Imperator, the Ork Mega Gargant, Chaos Titans and Imperial Knights.
These rule systems were again simplified and condensed and re-released as the third generation, Epic 40K. Epic 40K did away with a lot of the complexity of the system in favour of faster gameplay. The game engine had previously been developed for the Battlefleet Gothic game, where it had proven itself quite adept at keeping the pace of the game up even with a large playing force.
Epic 40,000 is set to be revamped once again in 2003 with the release of Epic Armageddon. In a number of articles and essays, Andy Chambers has explained how the new system will introduce some of the detail that had been lost in Epic 40K. It remains to be seen whether this, the fourth incarnation of Epic, will finally find the right balance, or will turn out to be just another money-spinner.
- http://www.games-workshop.com/40kuniverse/epic40k/assets/Epic7_02_09.pdf - Andy Chamber's essay on Epic Armageddon