DK's are tank/dps hybrids... beastly DPS, and very good avoidance tanking. They tank without a shield by stacking dodge and parry.
One or Two-hand dps, cannot equip shields, wears plate, has pets, though not in the hunter or warlock sense, if you spec correctly you can get a pet that lasts permanently but you need a reagent to summon them. You can spec for a gargoyle pet that lasts for a minute or two in combat and casts from above.
No mana/rage/energy, they use Runic Power, certain attacks give runic power (it doesn't regenerate like mana/rage/energy) and you have 6 different runes, two of each type Unholy, Blood, Frost, and certain attacks take X Runic Power + X Unholy Runes, X Blood Runes, X Frost Runes. Those runes are NOT reagents. You always have them, but they have a cooldown once used. They show under your Health and Runic Power bars.
I haven't played mine like.. at all. So someone else may be able to tell you better how they work, that's just the overview of the basics.