You become a Death Knight after you reach lvl 80. It's sort of like how when you reach a high enough lvl in your profession, like Blacksmithing, when you can choose what to specialize in.

From what I've gathered by reading around, Death Knight will be able to raise from the dead anything that he kills, and can have more than one "undead pet" at his at any given time.

World of WarCraft's first Hero Class is a plate-wearing tank/DPS hybrid that works a little something like this: When players hit level 80, they'll be able to embark on a quest (similar in difficulty to the Warlock's epic mount quest, back before the level cap was raised to 70) that unlocks the ability to create a Death Knight character. The Death Knight starts at a high level (somewhere around 60 or 70, though Blizzard isn't certain yet), so you won't have to grind your way back up all over again. It's intended as an alternative, advanced class for end-game use only.

Rather than using rage, energy, or mana, Death Knights have a special "rune sword" displayed beneath their health bar, onto which the player can etch six different runes (choose between Frost, Blood, and Unholy). Different abilities require different mixes of runes, and using abilities consumes the requisite runes until a cooldown timer causes them to be available again. Stuff like talents, spells, and other specifics are still being finalized. So are key questions like what races can be a Death Knight (start a Gnome petition now) and whether unlocking the Hero Class on one server means you can create a Death Knight on any server.