Orks first appeared in the Rogue Trader rulebook and the Space Orks boxed set, containing eighteen metal models. These Orks were squat, powerfully muscled humanoids, wielding crude ballistic weaponry (usually black powder or other explosive projectile-based equivalent) and large 'stikbomz' or stick grenades; along with their helmets and other paraphernalia, these are apparently modeled on German equipment from World War I.
Additional rules, cultural details, and painting guidelines were given in the later expansions Waagh Da Orks and 'Ere We Go!. An explanation for the combination of brutal stupidity with relatively high technology was now found: Orks had been created, in times long past, as a warrior race by the long-vanished Brain Boyz. (This theme recurs in recent editions, the Orks having been created by the Old Ones to fight the Necrons in the war that shattered their galactic civilization that existed prior to the rise of the Eldar.) In early editions, the Snotlings were said to be the last degenerate remnant of the Brain Boyz.
As the game evolved, some of the clunkier (if more flavor-filled) rules regarding Orks were dropped, though their culture remained much the same. For example, 'Ere we Go contains several pages of rules for mobs (i.e., squads) of Madboyz, Orks whom even other Orks consider maladjusted. (It should be noted that an average Ork would be considered dangerously aggressive to the point of insanity in a human society). The Madboyz change their behavior unpredictably according to dice rolls whenever conditions change, for example, when an enemy unit charges into combat with them. A roll is first made to determine which subset of behavior is going to apply, such as 'Skitzo' and 'Manik', then another roll is made to see how this overall insanity manifests. The results vary from charging the closest enemy, with bonuses to their combat attributes ("Waargh! Dat makes me mad!") to phobic avoidance of all vehicles, even ones on their own side ("Urr! Keep dem wurrin' teknikol bitz away!") While often very amusing, this was highly time-consuming, and dropped in the later editions, which were streamlined for combat.
The main background change lies in their method of reproduction. According to Waagh da Orks, Orks are born to feral enclaves as Wildboyz. They are then recruited into warbands and fight for a while with primitive weapons such as spears, before being recruited into a Boyz Mob and being given their first gun. If they survive the ensuing ten or twenty years of warfare, they become overwhelmed with the urge to go off 'lookin' fer sumfink' and wander back to their enclaves to breed, developing sexual characteristics on the way. In newer editions, however, adult Orks are constantly giving off spores which lie in the ground, often for years, waiting to develop into Orks or Gretchin. Thus a world invaded by Orks will be troubled by them for hundreds of years to come, even if the original assault is beaten off.
The beginning of the Orks has also been changed with the arrival of the 3rd Edition of Codex: Necrons. Here a xenos species called the Krork is described as a hardy green-skinned race, created by the Old Ones to defend their last strongholds against Warp-spawned horrors like the Enslavers. The C'tan called the Deceiver once expressed its surprise that Mankind and the Krork had spread across the galaxy. Since humanity and the Orks are the most numerous races in the Warhammer 40,000 universe, the connection is obvious.
Ork weaponry is built by the Mekboyz or Mekanikz, as well as being captured from enemies or delivered as tribute. It is intentionally portrayed as being rather Rube-Goldberg-ish, often steam-driven and with clunky, early-industrial driving belts and ad-hoc armor and extraneous bells, whistles, and gongs. It is often based on teleporter and force-field technology, which the robust Ork physiology can use more easily than humans; a weapon found in 'Ere we Go, for instance, is the Snotling Teleport Gun (also known as the Shokk Attack Gun). Snotlings are a tiny, Orkoid race even smaller than Gretchin and less bright than Orks, thus they are not usually of much use in combat. The weapon sends the Snotlings through a tunnel through a daemon-infested alternate dimension know as the Warp. The horrors the Snotlings experience drive them mad, and they emerge inside enemy power armor or vehicles as a biting, scratching mass. The Teleport Gun suffered from severe inaccuracy, but could be quite effective when it did hit. However, the rules for it were very complex, and it was removed from later editions.
In their current incarnation, Orks are very much a close-combat-optimized species, their weapons having short range and low accuracy - though this can sometimes be made up by sheer numbers. Orks are quite cheap in terms of in-game points, that is, it is possible to build a very large army, colloquially known as a 'sea of green' or 'green tide' from the skin color. This is often necessary, since the effectiveness of an individual Ork at any but the shortest range is small, and the Ork commander must be prepared to absorb considerable casualties in his effort to close in for axe ('choppa') work. This is in contrast to their earlier editions, where the large number of fun, but inaccurate weapons and special rules could easily make them rather unfocused, a jack-of-all-trades army but master of none. Some have accused the designers of regarding the Orks more as comic relief than as a serious army, and being more concerned with the flavor than the effect of weapons designed for them. If so, they have reined in their imaginations considerably where Orks are concerned, drawing the race more into line with the overall gritty feel of WH40K.
The current Codex: Orks was released at the tail-end of 4th Edition, on the 12th of January 2008. Despite the release date, most players view the Codex (alongside Codex: Chaos Daemons) as having been written with 5th Edition in mind. The Codex was accompanied by the release of a number of new models for the Ork range, including redesigned Trukks and Warbikes. The Codex also added units and equipment to the Ork army, many of which had been featured in the 2nd edition and removed for the 3rd. This includes the Shokk Attack Gun and the Weirdboy.