Dawn of War 3 is an upcoming real-time strategy game set in the grim darkness of the Warhammer 40,000 universe. It is the third main game in the Dawn of War series, though the actual gameplay is styled after the original Dawn of War with a heavy focus on base-building and absolutely massive armies.
If you're wondering what all of this might look like in action, and what the story is going to be about, here's the most recent "Prophecy of War" trailer. There's not much in terms of spoilers in here, so even if you want a completely 'pure" playthrough feel free to watch the video. Anyway, have a look:
Besides all of the obvious gameplay and story related stuff, two things stood out to me from this trailer: the presence of Imperial Guardsmen, and seemingly Necron architecture. So far Relic has only confirmed three factions (Orks, Space Marines, and Eldar), and I highly doubt they would keep any extra ones secret given how much more publicity they could muster with them, but it might just be possible that the Imperial Guard and Necrons will feature prominently in the singleplayer campaign. I certainly hope so, because it also means they might eventually make their way into the multiplayer mode as well!
All this speculation aside, if you're confused about the general Warhammer 40k lore and are currently wondering what kind of nonsense I'm even blabbering about, you can find a brief developer-guided story overview right here. The video is mostly geared towards beginners, so if you already know the faction leaders chances are you know pretty much everything about the story so far. And if not, the video's well worth giving a look.