Death Knight Talents and Skills.

Q: Which tree is the Tanking tree?
A. All of them, all trees of the Death Knight class are intended for Viable, competitive tanking capability, what the trees mainly come down to is preference of style and moderate gimmicks.

Q: Which tree is the DPS tree?
A. All of them, all trees of the Death Knight are intended to be viable, competitive dps, the trees are about style preference and gimmicks, it is where you put the talents in the trees that decide your role, not which trees you put the talents in.

Q: Can a Death Knight summon a pet/minion?
A: Yes, but unless you put a point in master of ghouls (unholy talent) it will not be controllable and only available for a couple of minutes with a 5 minute cooldown. After putting a point in master of ghouls the pet is controllable and permanent unless it dies.
Credit to Andyishere

Q: How do i summon a ghoul?
A: You get to level 56 and learn the spell 'Raise Dead'. Once you have done that, you need to find a corpse that is within your level range (as in, if you killed it would give you XP etc) and a humanoid. Otherwise, you will need 'Corpse Dust' which can be bought from any reagent vendor.
Credit to Andyishere

Q: 5 minutes sounds like a long time for a pet summoning ability to cooldown?
A: Yes it is, thank god for the talent Night of the Dead (located deep in the unholy tree), this reduces the cooldown of Raise Dead by 30 seconds per Scourge and Plague strike, and the cooldown of Army of the Dead by 60 seconds.

Q: What is Army of the Dead?
A: Army of the Dead is a somewhat hilarious answer to a tanking "OH GOD IT'S GONNA HUUUUURT!" cooldown. As warriors, paladins and druids will tell you, hunters that leave growl on in instances are really annoying, so our major "Oh crap." button is an Army of hunter pets with growl switched on and 3k hp, negating at least 8 melee swings that would otherwise be on the tank. While channelling this ability incoming damage is reduced by a percentage equal to your combined dodge and parry chance.