What is the Death knight's Role in PvE?

Death knights are a tank/DPS hybrid. All three of the death knight's talent trees are capable of both tanking and dealing damage in an endgame raid environment - though the player will want to pick more tanking talents over damage talents if he intends to fill a tanking position.

The 'niche' of the death knight is it's ability to tank spellcaster-type targets. All three talent trees, as well as base non-talent abilities, give the death knight more spell damage reduction and interrupts than the other three tanking classes, making them excellent tanks for any mob or boss which uses magic heavily.

Death knights will also perform effectively in a raid DPS role. Their personal damage output may not be as high as that of rogues or fury warriors, but they offer both their own damage as well as many useful buffs to the raid to increase the damage or survivability of others.