Tauren DKs represent

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Becoming a death knight does not, ironically enough, involve actually dying.
To become a death knight, a Paladin must vow before Ner’zhul (or one of his designates) to serve the Scourge, be accepted, and then receive a vampiric runeblade forged specifically for him. Once the runeblade is used to corrupt his holy powers and turn a hero into a death knight, it becomes that knight’s personal weapon and is slowly charged with the life energies of those it injures and slays thereafter.

That's a second generation Death Knight.

And they were killed, actually. It just wasn't a death you might think. The runeblade slowly drains their soul, their life, away. There's no threshold for when they go from living to dead, it just slowly happens.

3rd generation (WoW's) DKs are killed and reanimated.

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When Prince Arthas Menethil of Lordaeron gave himself over to Frostmourne during the Third War, he became Lich King Ner'zhul's first death knight, sworn to the Scourge and invested with dark powers to carry out its will. Since then, paladin warriors who accede to the call of the Scourge and make a pact to serve the Lich King receive a vampiric runeblade specially crafted for them and are anointed by Ner’zhul as one of his death knights. Thus the Lich King fashions his corrupted paladins: his greatest champions — living or undead — in his campaign to conquer all of Azeroth.

Much as Arthas had before them, these renegade paladins succumbed to bitter hatred over the course of their grueling quest. When they finally reached Ner'zhul's icy fortress in Northrend they had become dark and brooding. The Lich King offered them untold power in exchange for their services and loyalty. The weary, vengeful warriors accepted his dark pact, and although they retained their humanity, their twisted souls were bound to his evil will for all time. Bestowed with black runeblades and shadowy undead steeds, death knights serve as the Scourge's mightiest generals.

---------- Post added 2011-10-12 at 04:57 PM ----------

Never knew that. How can races that can't be paladins be death knights then? Is there some explanation somewhere?

It's just as impossible as having 2 dk's on one realm: it generally is, unless you're willing to pay for it, then it's possible all of a sudden. 2dk's on one realm by paying for character transfer, races that can't be paladin being dk by racechanging.