How does the Death Knight fight?

They don't have mana or rage or energy. They have a new system. They use Runes and Runic Power.

They have 6 Runes -- 2 Blood Runes / 2 Frost Runes / 2 Unholy Runes
The vast majority of their abilities cost a rune to be used. Some cost 2 or even 3 runes.
When used, a rune takes 10 seconds to refresh and be usable again.
When you use an ability that consumes runes, it generates Runic Power. Between 10 and 25 Runic Power per ability, depending on how many runes are consumed. You can also get extra runic power via certain talents.

Runic Power -- Pretty much like the warrior rage bar, you start with zero runic power, and can have a maximum of 100. You can use runic power to cast abilities such as Death Coil.