A race from Warhammer 40k. Also known as "greenskins". They have a Warhammer Fantasy equivalent, the only major differences being the more common spelling of Orc with a "c" instead of a "k", and a lower level of technology, related to the setting. Orks are commonly believed to be stupid and superstitious by the other races of the 40k universe.
Orks could be considered one of the more successful races of the 41st millennium. Despite their entire lack of structured education or training, they seem to be very proficient with all kinds of technology, which they inevitably utilize for their armaments (of which firearms and vehicles are the most common). They are often very resourceful, able to have the minor greenskins, such as "Grotz" (goblins), construct several working vehicles and machines out of mere scrap. Common Ork weapons are a choppa and a slugga.
A large contributing factor in the Orks' success is their ability to reproduce extremely fast because of their fungal-like existence. This means that an Ork will spread several spores in his life time, each of which is able to grow into another Ork, given enough time. Orks as such breed through asexual reproduction.
The fungal physiology of the Ork is also why they are so tough as they are, since their body has fewer vital organs.
OMNOMNOMNOMNOMNOMNOM Delicious Biomass, you must devour it.
Tyranids are extragalactic beings in Warhammer 40k that exist only to eat living things. They have been expected to arrive on Terra's doorstep any day now for years, being stalled by a force even more malicious then they are: GW's reluctance to move the story forward (now with painfully adorable theme song, which is totally not a Rickroll: http://www.youtube.com/watch?v=SMWi7CLoZ2Q).
Some fa/tg/uys masturbate furiously to anthropomorphized versions of Tyranids.
Oh yeah, they exist to eat GW's errors
The Tau are a playable race in Warhammer 40k. When first discovered by humanity, the Tau were a barbaric and primitive people. Then their planet was trapped in a warp storm for a few thousand years and they emerged from the other side as a unified species, led by the mysterious Ethereal caste and devoted to the concept of the "Greater Good". The Tau were originally developed because Games Workshop felt their setting needed an optimistic race, making the Tau the least grimdark faction in the game. Somefluff has them sterilizing races that resist the rule of the Tau Empire, and their Codex leaves ambiguous how much of their success is due to various forms of mind control, within and outside the Tau themselves. It says something about 40k that despite this they're still the friendliest race in the setting.
They are a technologically advanced race, and have some of the most powerful ranged weaponry in the game. Tau units in general are rather weak in close combat, so they rely on alien auxiliaries such as the Kroot and Vespids who have joined the Tau Empire to bolster their strengths in this area. Fire Warrior veterans and commanders go to war wearing advanced battlesuits, allowing them to field even more firepower.
Tau unit designs were somewhat inspired by Asian culture and Japanese mecha. As a result, /tg/ declares the Tau are weeaboo. Their devotion to the philosophy of the Greater Good often leads to them being labeled as communists, too.
 Notable Tau
Eldar are spess elves.
The INTERSTELLAR ROCKSTAR version of the eldar, which also happens to be their polar opposite. The dark eldar are a race of evil space elves who fly around the galaxy killing, fucking and torturing anything sentient. This is in sharp contrast with the totally hetero armies of flamboyantly dressed dudes that hang out exclusively with each other and work out, and the all-male armies of guys with father issues that live in a cosmic anus.
 Spess Mehreens
The Space Marines are genetically engineered supersoldiers clad in powered armor, and are generally regarded as the toughest bastards to ever serve the Emprah. The average Space Marine is around eight feet tall, has bones that can repel present-day small arms fire, can breathe poisonous gas with no significant problems, is capable of spitting acid, and lives for hundreds of years.
The Space Marines themselves were created by the Emperor using genetic information from the Primarchs, who were also created by the Emperor but subsequently kidnapped by the ruinous powers of Chaos and scattered across the galaxy while still children. All but two of the original twenty Primarchs were later recovered and reunited with the Space Marine Chapters their genetic material had founded.
Space Marines are generally regarded as something of a "noob army", mainly because every twelve-year-old 40k player has a raging hard-on for them, and almost every unit in the listings has at least a 3+ armor save, making them rather hard to kill. The Ultramarines in particular are an extremely popular choice of Space Marine Chapter, and their blue design coupled with the small size of the miniatures often leads to them being referred to as "Smurfs". Thanks to Indrick Boreale, the Space Marines in general are frequently called "SPESS MEHREENS", or variations to that effect.
Anyone who doesn't play Space Marines is either a Tyranid, a faggot, or more than twelve years old. Probably all three, since no one cares about armies other than the aforementioned ones, and 'nids are totally fabulous. Also, twelve year-olds play WARMACHINE as hard as they can.
The Space Marines of today look very different from the glory days of Rogue Trader, where they earned the nickname "Beakie" for their fuckawesome helmets. GRIMDARK fluff was less prevalent, and Mahreens were psychotic killing machines, not monks for TEH EMPRAH.
 /tg/ Space Marine Chapters
One of /tg/'s favorite pastimes is creating new and exciting Chapters of the Adeptus Astartes based on silly concepts. /tg/'s homebrew Chapters include:
 Chaos Space Marines
Chaos Space Marines are to the Space Marines what the Dark Eldar are to the Eldar, that is to say EVIL SPESS MEHRENS!
One day, over 9,000 years ago, (I think it was 10,000) Horus the Primarch decided "I HAET EMPRAH AND LOEV DELICIOUS WARP TAINT HERESY!" It was called the Horus Heresy. Half the Space Marine Legions followed Horus and started worshiping the Gods of Chaos: Nurgle, Khorne, Slaanesh, and Tzeentch.
They fought a big war, with the Chaos Marines fucking shit up all the way to Holy Terra. Oh man, it was so awesome, there was blood and guts and shit everywhere. SOOO grimdark, but then the Emprah killed Horus, and the Chaos Marines went home, which was now the Eye of Terror. Then the Chaos Marines sat around and replaced all their iconography with more Chaosy bitz (like instead of skull depictions on their armor, they just use human heads. Spikes, horns, chains, and symbols of Chaos are also good too. Bonus points for mutations). And every once in a while, they launch a Black Crusade that ends in failure.
Supposedly, if you read the fluff, these Chaos guys are still largely the same dudes who defected to HERESY! 10,000 years ago. Which seems pretty badass; until you think about it. Really, it's sad, considering these sons of bitches are 10,000 year vets and are on Black Crusade #13 and they accomplish absolutely DICK each time. In the grim darkness of the grimdark future, there is only grim darkness, dark grimness, and STALEMATE.
 Imperial Guard
The Imperial Guard makes up the bulk of the Imperium of Man's military. Unlike the
Spess Space Marines, they are numerous, usually poorly armed, and poorly trained, and prone to dying for the Empra (whether facing towards the enemy, or not). Depending on the setting, they can either be competent and courageous, or cannon fodder that make the Space Marines look awesome in contrast.
They sometimes manage to kill the enemies of the Imperium with flashlights, but they usually use tanks. Like a shit ton of tanks. And badass grandpas.
Also they fall on their heads.