The Warhammer 40k universe is HUGE. And knowing is about 9/10ths of the battle. To facilitate understanding, I have broken down the important parts into Layman's terms for the rest of you to read over. This way, whenever Cain is like "DERP", you as a player can go "HERP" and we all get along.
Eldar - SPACE ELVES - aka pointy ears, limp wrists - The Eldar are a race of beings who used to rule over much of the galaxy. Unfortunately, they got so perfect they started doing things to relieve their boredom. Horrific things that spawned a Chaos God which killed off the majority of them. Even so, they're long lived and many have strong psychic powers. The Eldar believe they are better than everyone else and see other races as young upstarts. Eldar do not hesitate to slaughter other races for simply being in the way of their objective of maintaining status quo. They're often considered girly to humans because their voice is on a higher wave length, making them all sound like females to human ears. Also, they have thin, shapely bodies, adding to the effect that they're all girls.
Tau - Weeaboo Space Communists - aka bluies, grayskins - The Tau are the most technologically advanced race in the Warhammer 40k universe. They're also the youngest, being around 6,000 years old. They lack psychic abilities and have had no contact with the Warp, which may be the reason for their sheer advancements in science. The Tau are also the...nicest race in the 40k universe. Instead of killing everyone in their way, they offer their foes the chance to join them and kill them if they refuse. The Tau believe in the “Greater Good”, which is the concept of putting the needs of the whole over the needs of the individual. Despite being “nice”, they have a very 1984 style thought police system where if you have doubts about the Greater Good, you may find yourself “missing”.
Necrons - SPACE ZOMBIE ROBOTS - no nicknames - One of the oldest lived races, they live a life of nothingness. They used to hate the fuck out of everyone but were really weak. Then some crazy star eating gods tricked them into becoming robots and now they just kill every living thing in existence. The Necrons also lack psychic abilities so the Eldar are their worst enemy. Whenever Necrons show up on a planet, that planet is usually destroyed to keep them from spreading around the area.
Orks - SPACE.....orcs - aka greenskins - Possibly the coolest race in the 40k universe. They're all part of a psychic semi-hivemind when in close contact and generally believe in beating the poo out of the nearest thing. When there is a strong enough ork, he can gain control of the psychic semi-hivemind and guide the orks into a WAAAUGH!!! and start huge fights over planets. They speak a slumlike British English and often refer to things as an onomatopoeia or childish terms. Red also goes faster. They have two gods, Gork and Mork, who are cunningly brutal and brutally cunning (one punches you when you look and the other punches harder when you're not looking).
Tyranids - prototype Zerg - aka 'nids - Think the Zerg but only 80 billion times worse. The Tyranids swarm through the universe and are almost unstoppable unless the brains behind the swarm can be found and destroyed. Even then, they'll continue attacking until they're completely killed. Fighting them is nearly impossible since they can process any existing lifeform into another Tyranid through bioprocessing. Theory has it that they come from another galaxy which they had already wiped out of lifeforms.
Humans - RELIGIOUS FANATICS IN SPACE - aka so many unspellable names, humies - Humans are absolutely batshit in the 40k universe. They worship the Emperor, who is a corpse god, and hate everything that does not worship said Emperor or anything that isn't human. Mutants, heretics, and aliens are generally shot on sight. Science to them is magic and attempting to innovate AT ALL is punishable by death. Humans are currently the main power in the 40k universe but are losing badly to the Tyranids, Orks, and Chaos. It's thought the Tau will be the next super power in 40k.
Chaos - HERETICS IN SPACE - aka heretics, warpspawn, scum - The Chaos revolves around the 4 Chaos gods and represents anything that is change. The gods generally fight amongst each other but when the four gods can get along for a singular reason, they tend to explode out of their portion of the galaxy in a Black Crusade. Aside from that, it's easy for a human to fall to the grasp of Chaos and happens very frequently due to man's unstable nature. The forces of Chaos are generally made up of Daemons, fallen Space Marines, and fallen humans.
SLANG TERMS AND DEFINITIONS
Astartes - Adeptus Astartes - SPACE MARINES
Autorifle - a military grade rifle
Bolter - a gun that shoots exploding rounds much in the fashion of an RPG. Comes in pistol form too.
Emperor - the god of mankind. Used to be living but now is in a type of stasis but dead.
Emperor-botherer - someone who is fanatical about their devotion to the Emperor.
Enginseer - a mechanic
High Gothic - the sacred language of humans. Practically unintelligible to most in the 40k universe, it's usually represented by a Latinized English.
Imperial Guard - the army of the humans. Its numbers are so large, it's uncountable and is well into the trillions. Generally considered cannon fodder due to being easily replaceable.
Laspistol - a pistol that shoots a laser
Lasrifle - a rifle that shoots a laser
Low Gothic - the language of humans. Generally a British English with a basis in Asian languages and German.
Recaf - coffee
Stubbers - one shot guns such as bolt action rifles, pistols, revolvers, etc
Technosorcery - the term used for science as it is treated as a type of magic for humans.
Techpriests - a broadly used term for any person who has devoted their life to the magic of technology.
Warp - the immaterium. Beings such as dark gods, daemons, and hosts of other evils live in the Warp. It's also used to travel faster than light speed.
More will be added soon as soon as I'm reminded or remember. ALSO SPELLING PROBLEMS.