The Warhammer 40k universe has been going on for nearly thirty years now, and as such its a bit ambitious to expect everyone to know what in the blazing hells is even going on. Sure, I'd wager most people know what an Ork or a Space Marine are, but what about the specific characters and their motivations, or even the differences between the various factions?
So if you belong to the "I have no idea what's happening" camp and would like to learn a little bit more about Warhammer 40k before Dawn of War 3 arrives, you'll be glad to hear that the developers have recently posted a brief overview of the general lore and the characters you'll meet. Have a look:
And if you're interested in some actual gameplay footage, the developers have just recently posted a preview of all the different units and their 'roles' in the Orkish army. You can find it by heading over here. As for the release date, Dawn of War 3 is set to arrive at some point in 2017, though no exact date has been given just yet.