Epic
A system for running massively huge (or, one might say, EPICally huge) games of Warhammer 40,000, using streamlined rules and 6mm scale figures to make things feasible. Forgeworld never bothers to stock shit for this anymore since its been overshadowed by its big brother Apocalypse. This is purely a game play decision and has nothing to do with the fact that the 28mm scale resin models in Apocalypse are outrageously expensive and supported by special books you have to buy.
In the Before Time
Epic began as a system by the name of Adeptus Titanicus and was all about giant-ass Titans fucking each other over with huge weapons and stamping on puny infantry. Then, along came a system called Space Marine: Epic designed to allow more rounded armies and hailed as a way to show the true scale of Warhammer 40k conflicts, seeing as the fluff demands wars on the level of the Apocalypse over a black Templar spilling iced coffee on an Eldar Guardian. As this was during the Second Edition phase of design, it did this via a diseased combination of "Army Cards" where building an army was a matter of creating a spreadsheet with a bunch of dead trees. In this system the majority of models were 1/350 scale, while large vehicles were 1/700 scale to keep them affordable (seriously, there was a time when Games Workshop actually thought like this). This also got GW the Space Marine registered trademark in the UK, which they took great joy in forcing figures based on the Aliens franchise to acknowledge even though the second movie came out first.
Old Epic had some strange issues with armament; specifically, weapons were divided into broad types with no real acknowledgement of different subtypes; all "bolters" (pistol, bolter, storm, heavy) had the same stats, for example, and the Battle Cannon in a Baneblade's turret had the same stats as the one in a Leman Russ', or the one in a Stormblade's sponson.
How it worked
An Epic army was organised using army list-like groupings based on cards with point values. Each subunit of the army was formed of one Command card, which could take up to three Company cards, which could in turn take a Special card. Command cards were typically large infantry formations, Company cards infantry or mechanised infantry, and Special cards the superheavy vehicles, Titans and suchlike. Each of these cards also had a point value printed on it, and as usual armies were limited to a number of points agreed on by the players.
Titan Legions
At some point, however, someone realised that Space Marine meant that the average army contained loads of cheap plastic minis and only a few big, expensive lead Titans (nevermind the stupid super-vehicles like the Capitol Imperialis and Hellbore that nobody bought or used at all and nobody remembers). This led to the creation of Titan Legions, which introduced Titan Companies as support cards, a pair of new super-Titans for the Orks and Imperium (with more planned) and the Imperial Knights, which entered their 2-year cycle of GW forgetting and remembering they exist at this point in time. It also marked the start of the fluff acknowledging that Orks actually had aircraft.
Titan Legions also introduced greater support for large units of armour as Company cards, and the Tyranids as a playable army; written in concert with Codex: Tyranids for 40K, this was the first time it was acknowledged that any Tyranid creature larger than a Carnifex and smaller than a Hive Ship actually existed.
Predictably this failed to really do anything; Titan Legions was underwhelming and didn't address any of the issues with the core system, instead just adding an extra dollop of complexity on top of it. Most of the new minis were never released at all, while some planned for later supplements like the Ordinatus weapons only ever had their rules featured in White Dwarf. Being the only place the Squats didn't suck made the Epic system an obvious candidate for the chop, and soon the whole line was discontin
The Other Other Other Epic
Eventually a newer, streamlined system called Epic 40,000 came out. It was well-designed and the rules made sense, and the players who bought it (both of them) saw that it was good.
When Somebody Said:
"Hey, you know what would be awesome, making 40k scale models to match the level of scale Epic was trying to make."
And thus began the slow spiral into madness. Don't get me wrong, I love EXSBAWKS HUEG models and I make them, but Epic was a relatively rules-easy game that had a solid fanbase that died to Apocalypse, which was poorly designed and is cumbersome to play. There is now a new system called Epic Armageddon, which GW barely supports with a range of old Epic 40,000 models and nobody cares about.