Yo, if you want a primer into W40K lore, here's your primer into W40K lore;
Space Marines = Space Nazis. Also, they're all clones. Also, they can't remember how to do technology any more.
Eldar = Space Elves. Also, they're all arrogant perverts. Also, they managed to fuck a chaos god into existence because they couldn't keep it in their pants, so their sous get eaten when they die.
Orks = Space Orcs. Also, they're the comic relief. Also, they're a fungus powered by the power of wishful thinking.
Chaos = Space Marines going through their teenage angst phase. Also, definitely listen to Linkin Park.
Imperial Guard = All the red shirts from star trek got together with phasers set to stun and figured they could help. Also, are irredeemably fucked in any battle scenario you can think of.
Everyone else = not popular enough to worry about for at 3 expansions.
This is so wrong in so many levels.. I just have to chime in and give an actual summary of the races..
Space Marines: Genetically engineered warrior monk supersoldiers equipped as basically walking tanks and organized into chapters that serve the Emperor. These chapters range from selfless defenders of humanity to singleminded forces of destruction. The forces of the Imperium of Man at large as much fear and abhor them as they need and envy them.
Eldar: An elder alien race who has been a part of the galaxy longer than most others. Their psychic abilities and hedonistic tendencies combined to gave birth to a Chaos God, tore down the barrier between the Materium and the Warp and basically fucked up the whole galaxy. They have since splintered into two basic factions, one who still pursue their sadistic and hedonist urges, and one who sulk and watch everything from afar and self-righteously meddle in the affairs of 'lesser' races.
Orks: Space Orcs. Comic relief. Genetically created militaristic alien race who were meant as an organic weapon against Necrons, they went out of control and became a menace against the whole galaxy. Ironically, they are quite possibly the galaxy's most effective defense against Tyranids and Necrons.
Chaos: Forces of the Warp. The Warp is basically an alternate dimension of psychic energy. The emotions of sentient and psychic races in the galaxy have fucked the Warp up completely with overwhelming feelings of rage, pride, fear, lust, and gave birth to Chaos Gods. These Gods (and their daemons) are basically invading the material world in pursuit of acts that will generate emotions to further empower them (i.e. war, pestilence, decadence, what have you). Some Space Marine Legions succumbed to the powers of Chaos 10,000 years ago and continue to wage an eternal war against the Imperium.
Imperial Guard: This is basically the Red Army of the Imperium, i.e. poorly trained, poorly equipped Imperial military juggernaut that relies on throwing enough corpses at the enemy until they die under the pile of bodies. "Even the man who has nothing left has his life to offer" is pretty much the Imperial Guard's summary.
And then there are the Tyranids who are an extragalactic alien hive whose sole agenda is to consume as much organic material and assimilate genetic code of enemies, the Tau who are busy founding an Empire for the Greater Good which is actually not as good as it sounds and would probably be crushed by the Imperium in a heartbeat if it actually grew to a size even remotely great, the Necrons who are basically a once sentient great race who transferred their consciousnesses into robots after being misled by lesser gods called C'tan, finally awakening from millenia of sleep to reconquer the galaxy with terrible weapons for their own alien purposes, and so on and so forth..
Anyway.. It's just not as cartoony as presented, so there you have that.