WotLK Death Knight questions intensify*

Death knights are the Scourge's dark warriors, defeated heroes risen to fight those same people they defended in their lives and with their lives. The 3 main aspects of a death knight are mastery over blood, frost, and unholy. All three specs use plate armor and are capable of both DPSing and tanking. Mastery over blood is manipulating the enemy's life force and your own so that you're always on top; so that you're sucking the life from them to fuel your vampiric strength and crush them. As tanks, they use their vampiric vigor to sustain themselves and outlive their opponents. Mastery over frost is freezing the enemy solid and cutting him down with icicle-sharp strikes, bringing him a cold white death. As tanks, they are as solid as a block of ice, resisting attacks and spells alike. Mastery over unholy is delving deeper into the essence of undeath to summon undead minions to obey you, and spreading terrible incurable diseases to melt down your enemy's flesh. As tanks, they use necromancy to strengthen themselves, with bone barriers protecting them from harm, and anti-magic abilities - from spells in particular.

Before we start, you have to know that DKs have 2 resources - a rune set made up from 2 blood, 2 frost and 2 unholy runes (which I will refer to as BBFFUU), and runic power (RP), which is generated by using those runes. There is also a fourth type of runes, called Death runes, which are generated through abilities IF you have certain talents (each tree has one such talent).

Blood and Unholy play with a 2-handed weapon, while Frost plays with dual-wielding one-handed weapons. The style of the specs is quite different from one another. Blood is a very self-sustainable tree, which makes it the best choice by far for leveling, it deals primarily physical damage, and it focuses on the very brutal and spammable Heart Strike (1B) and the strong damage- and healing-wise Death Strike (FU), while also being able to summon a copy of your weapon to mimic your attacks (60 RP). Your main runic power ability as Blood is Death Coil (40 RP), which is rarely used though because you'll be too busy using rune abilities instead. Frost is a fast-paced spec with mixed physical and frost damage, coming from the very strong Obliterate (FU) and Frost Strike (40 RP), which is also the spec's RP dump ability. Frost also allows you to literally freeze all of your enemies around you for a short time (60 RP), or to encase your armor in ice giving you more strength and defense. Unholy is unique in that you have a permanent ghoul pet (Blood and Frost can only summon temporary ghouls), which serves to protect you and deals quite some damage. It's not a tank in the way hunter pets are, but it offers good utility and support. If you need to, you can even sacrifice it to heal a large portion of your health (all 3 specs can do that, but as Unholy you always have the ghoul next to you ready for sacrifice). The spec greatly enhances your diseases and Death Coil, which is the spec's RP dump (only this time it's a lot stronger than if you're Blood). It also provides some unique tools, which make it the overall best spec for PvP (an AoE slow, anti-magic zone, disease-dispel prevention). The spec's main runic ability is Scourge Strike (FU), which deals part physical and part Shadow damage. Last but not least, Unholy can also summon a gargoyle to fight for you which deals significant spell damage for its duration.

I'd recommend starting out as Blood, as it is the easiest spec to use while leveling. Once you get to 80lvl, you can experiment and see which of the 3 specs you like the most.