Any class can unlock the deathknight class. Once your character is the appropriate level (55, I think), you are able to create a death knight character.
A deathknight uses 6 runes to cast spells. Each rune, after use, regenerates to be available for reuse after a period of time. There are different runes for each tree, so it's almost like having 3 different mana pools. Using spells that require runes builds up "runic energy," which can then be used to cast spells which require it.