What are the death knight specs?
Blood is the death knight tank spec. If you want to tank, this is the talent tree you want to grab. The blood spec helps pump your health to high levels and gives you various ways of draining and recovering health in order to survive taking hits from mobs. This makes it pretty decent for leveling, but if you're in a group, you should only spec blood if you mean to tank. Otherwise, go frost or unholy.
Frost is a DPS tree that focuses on heavy-hitting weapon strikes with a bit of backup frost damage. The tree supports both two-handed weapons and dual wielding via talents that provide certain benefits to one weapon type or the other. You'll mostly be focused on aligning procs and buffs in order to churn out huge hits with your Obliterate, although you also get a very handy AoE frost damage tool in Howling Blast. Go with this spec if you want to DPS but don't want to deal with a pet, if you want put out some huge damage numbers with your weapons, or if you want to dual wield.
Unholy can almost be considered a caster DPS tree in a lot of ways. While you still do a lot of physical damage, your diseases are made even more powerful, you'll be casting a lot of Death Coils, you can apply a magical damage debuff to the mob, and you get a permanent Ghoul pet with a lot of unique abilities that you'll want to learn how to use if you want to play an unholy death knight to its full potential. In addition, Unholy has a somewhat unique damage rotation, because your main damaging weapon strike uses only one unholy rune, as opposed to the other two specs, which use an unholy and a frost rune. One of the biggest disadvantages to this is that it's a lot harder to fit in an emergency Death Strike for healing like it is with the frost tree. The tree is designed to use a two-handed weapon, although unholy DPSers have often tried dual wielding, only to have Blizzard nerf it within a patch or two.
What do stats do for me as a death knight?
Hit rating In order to be good a DPSer, you need to be able to hit stuff. Primarily, your weapon strikes (such as Scourge Strike or Obliterate) need to be able to hit stuff. For the first tier of Cataclysm raiding, this means you need a hit chance of 8 percent. Frost dual wielders will get 3 percent of that from talents, but Frost two-handed weapon users and unholy DPSers will need to get all 8 percent from gear. Now, if you inch a little bit above 8 percent, don't panic. You can reforge or regem to lose some of it, but if you're a bit over 8 percent, that hit rating is not wasted. It's going toward the higher hit cap for your spells, such as Death Coil. Don't worry about hitting the spell hit cap, though. You're better off stacking more strength once you get that initial 8 percent.
Tanks can get some use out of hit rating as well. The more your hits connect, the more threat you get. If you're having problems keeping threat, you may wish to consider trying to hit the 8 percent special attack hit cap, but it's not absolutely as mandatory as it is for DPS.
Strength Strength is the bread and butter of DPS and is the single most important stat for any DPS death knight. It gives you attack power, which makes your physical attacks hit harder. Your spell-like effects also gain power from attack power. Yes, this means you don't want Intellect to make your diseases tick for more; you want strength. If you have intellect or spellpower on your gear, get rid of it as soon as possible. It isn't doing anything for you.
Tanks benefit from strength as well, in that harder-hitting attacks mean more threat. Still, you should get the strength you need off of regular tank gear, so you don't need to worry about it as much as DPS.
Haste Haste makes your weapon swings faster at the most basic level. Unfortunately, it doesn't increase the speed of your DoT ticks like it does for other classes, but that is in part because it helps your runes regenerate faster. It will also affect your pets, including increase the energy regeneration rate of unholy's ghoul. 2H frost and unholy DPS death knights will find haste especially useful, and it's easily the third most useful stat for unholy death knights after hit rating to the cap and strength, in part because it does affect your ghoul pet, in part because it helps you throw out attacks as fast as you regenerate the runes for them. 2H frost gets use out of the haste stat because, again, it helps you unload all your attacks faster, and because faster swings means more chances for Killing Machine to proc. Dual wielding frost death knights won't make as much use of this stat simply because they're already global cooldown-capped. They already put out resources faster than they can spend them.
Tanks shouldn't really bother with haste. If you need more threat, get more hit rating, expertise, or strength.
Expertise With 26 expertise, you'll be able to keep the enemy from dodging your attacks. This is very important to frost death knights, who do a lot of their damage from big hits with Obliterate, so you're going to want to make a real effort to get to 26 expertise after you hit the 8 percent hit cap. Unholy death knights don't really need to worry as much about expertise. It doesn't affect your pets, and it doesn't affect Death Coil or your diseases, which means a huge chunk of your damage just doesn't need it. Get more strength and haste instead.
Tanks can also do well with that 26 expertise target. Again, it's not the end of the world if you don't reach it, but it helps your threat.
Critical strike rating Hitting harder with your spells and strikes is certainly helpful for DPS, but critical strike rating still falls out to be fair to middling for DPSers. Frost DPSers already have Killing Machine to give them a lot of critical strikes, making the stat itself almost redundant, while unholy death knights will always find haste or strength to be a better choice. Critical strike rating isn't something that's bad, per se, but the fact remains that there is almost always a better stat you could be using.
Much like haste and strength before it, tanks are better off getting other stats than critical strike.
Mastery You won't actually have to worry about mastery until level 80, but it's still worth mentioning so that you're ready for it. Between Frost Strike and Howling Blast, frost death knights actually get a decent amount of use out of frost mastery, enough that it comes in a bit under strength for dual wielders, and under haste for 2H weapon users. Unholy, on the other hand, just doesn't get enough of a boost from it, as it just doesn't boost their disease damage enough to make it better than other stats. Blood tanks definitely want as much mastery as they get, though, even going so far as to reforge avoidance to mastery. More mastery means their Death Strikes provide even more blocking protection via Blood Shield, which means they stay alive longer.
Stamina DPS death knights don't have to worry about stamina too much. You'll probably get as much as you need on normal plate gear. Tank death knights, though, should stack as much stamina as they can get their hands on. Stamina means more HP, especially when it's multiplied by talents, which means you survive longer. Stamina also feeds into Vengeance, which means it technically gives you more threat as well. Consider stamina your No. 1 priority and the all-around god stat if you're a tank.
Dodge and parry Dodge and parry or more or less worthless for DPS in the grouping game. You shouldn't ever be hit, so you won't really need them. Tanks, however, will find them useful, as every time you aren't hit is one less time you need use a healing cooldown or have the healer use their precious mana to heal you. That said, as you gear up to tank, you should probably be focused mostly on stamina and mastery. When you do get dodge and parry, try to keep your chances to dodge and parry as close to equal as possible, since each type of rating has equal diminishing returns. Having 25 percent dodge and 25 percent parry is better than having 30 percent parry and 20 percent dodge, if only because it means you're more efficiently dividing the amount of parry and dodge rating you have.