Watch the lore video (40k lore in a minute) posted above for anything you really need to know to play DoW.
The series has some of the best voice acting I've heard.
(unit quotes from the DoW1 series set to Requim for a Dream to make a song)
Dawn of War 2 voice actors. (The Hivemind voice actor still scares the crap out of me)
Basic, basic, playable race overview:
Imperial Guard: Your basic human, given paper armour and a flashlight and sent out to fight nightmarish horrors with only their balls of steel and infinite numbers to protect them. Think ww2 era Russian army. (IG tactic? "If it doesn't work, throw more men at it")
Bolter bitches Adepta Sororitas. Games workshop's bullshit answer to the legitimate question "but why can't there be female space marines?". They're actually incredibly badass but they get screwed over in black library books near constantly. Unmodified human females in power armour. Think Nuns combined with the Spanish Inquisition
Space Marines: Post-human killing machines created to be soldiers to fight for the Imperium of man. Originally there were 20 legions of them, 10 fell to chaos while 10 remained loyal. These have been broken up into smaller chapters. They are heavily implanted and have high tech gear available to them.
Chaos:
MAIM. KILL. BURN. MAIM. KILL. BURN. Humans, space marines and assorted aliens corrupted by the warp (a mirror universe that exists alongside our own used for FTL travel made of strong emotions and feelings where demons and evil gods live). Either servants of
malal, Khorne (god of blood, fighting and killing), Tzeentch (ambition, plots, scheming, sorcery, change), Nurgle (plague, sickness, decay, undeath), Slaanesh (Pleasure, pain, perversion) undivided (worship all four equally or undeclared). Tend to have crazy psyhic powers and mutations.
Weeboo space communists Tau. A naive race that a lot of people feel don't fit with the grimdark theme. Their goal is to have all races united under the concept of "the greater good" where everyone has their own place. GW responded to the complaints by grimdarking them up a bit, implying their Ethereal caste were actually mind controlling the other castes who are basically slaves.
Tyranids. Nightmarish creatures who want to devour the galaxy. They harvest all the consumed matter into biomas and use it to create more of themselves with a close to 0% waste ratio. Think xenomorph from aliens and add more teeth and more killy. Swarms of the little bastards eat through metal while the bigger ones shoot you or pounce and rip your face off. Not nice to be an Imperial Guardsman when these ugly gits show up.
Orks
is the biggest and the strongest. sentient fungus race that is taller even than a space marine, stupidly strong and
is made fer fightin' an' winnin' are warband like in nature.
Orks iz neva beaten in battle It
always needs more choppy, more killy and more dakka. Their tech is a bit basic but highly effective and works through a kind of group placebo effect; if the orks think that pile of scrap metal carved into the shape of a gun will fire, it will fire.
Necrons are terminator like beings that want to annihilate all organic life in the galaxy due to a tragic backstory. They are sleeping below the ground in tomb cities on some of their own homeworlds, some of which have been since repopulated in the milennia since they were last awake.
Except they aren't because they were recently ruined by a retcon. DoW is before the retcon so they are still their awesome exterminating selves.
Eldar: space elves. An old, powerfully psychic race, that once ruled the galaxy till they were so hedonistic and perverse that they actually birthed a new Chaos god (Slaanesh) in the warp. Slaanesh's birth cry tore a fabric in reality, claiming the Eldar homeworlds for the warp and consuming the majority of Eldar's souls. Currently living eldar live on space ships and "walk" "paths" to keep their minds fully disciplined and occupied e.g the path of the Seer. Each carries a soul stone that captures their soul when they die to stop it returning to the warp and getting omnomed by Slaanesh.
Dark Eldar: Perverted, souless, Eldar who are basically space pirates that kidnap and brutally torture their victims for kicks. Needs more spikey bits.
Chaos are the best LET.THE.GALAXY.BUUUURRRRN!
If you find anything you need defining then it's best to search it either at lexicanum [http://wh40k.lexicanum.com/wiki/Main_Page] which is basically a wiki of all the lore but is a bit addictive to read or if you can stand a bit of bad language, in jokes and slightly twisted humour 1d4chan [http://1d4chan.org/wiki] is a good wiki to use for laughs, strangely informative sometimes too.