Lets start off with some of the basics, Death Knights have three main talent trees Frost - Mainly used for tanking- Blood - Mainly used for DPS, think of it like a ret paladin just with a cooler name- and Unholy, which specialize in AoE and summons and good ol' DoT's.  Almost like a necromancer that has a two-handed sword. 

Another important topic to cover is weapon proficiencies.  It has been confirmed that Death Knights can use Two-handed swords, two-handed maces, one-handed swords, one-handed maces, polearms and one/two-handed axes.  There is some speculation based on weither or not Death Knights will be able to duel weild.  Early in the Frost talent tree Death Knights get an ability called Nerves of Cold Steel which increases there offhand weapon damage by 5% each point going up to 15%, this makes it quite clear that Death Knights will have the ability to Duel Weild, and sticking it in the frost tree makes it seems that Death Knights will almost certainly Duel Weild to tank.  

The next topic is armor, plainly stated by the devs Death Knights will use Warrior like armor stacking high in hit rating and ap.  Due to the fact that DK's dont need Spirit or Int because they dont use Mana makes it easy to stack purely on Ap/hit.