DeathKnight.info - Index
I normally go there for any sort of DK info, and for indepth theorcrafting I go to EJ.
They're a complex class and some of the mechanics are still only now coming to light but their primarily a DPS/Tank hybrid that inflict damage via melee attacks and diseases - the amount and virulence of said diseases depends strongly on which tree(s) the DK has invested in. If you look at the talent calculator DK's are a break-away from the oldschool "Tree X is for Tanking, Tree Y is for DPS" because there are viable DPS and Tanking talents in each tree - the only tree that really screams "THIS IS MY FORTE" would be Unholy for PvP and even then you can definately employ other builds with aplomb.
Their spells fall into two categories - Shadow and Frost (yes, there's some quite insane synergy with Frost Mages that may or may not be nerfed before live).
If you go here:
Wrath of the Lich King - Death Knight - Blood Skills
There's a talent calculator, as well as a breakdown of spells/talents from each Tree (Blood/Unholy/Frost).
The biggest stumbling block at the moment for many people is the resource management system of Runic Power (from 0 - 100) and Runes (every Deathknight has 2 Blood, 2 Unholy and 2 Frost runes. Earlier builds allowed you to customise this quota but led to insanely powerful builds - 5k+ dps Blood builds for example).
Abilities cost either a rune (or runes) or runic power. Runes, once used (aside from talents) refresh in 10 seconds and Runic Power is built up by using spells/talents which generate Runic Power - you can then dump runic power into
other spells and talents which are normally dependant on the amount of runic power available.
Diseases themselves are DoTs and depending on which spec you choose there are options to get from 2 - 4 diseases on a target. Some abilities deal damage based on the amount of disases on the target
Obliterate - Wowhead Search and some increase the damage/duration of these diseases
Epidemic - Wowhead Search.
I could waffle on for ages and not manage to illuminate you any further - you should definately go check out some links there and check out a couple of video's for a better idea of how they work.