Death Knights have a class-specific profession called Runeforging, similar to the Rogue-specific Poisons and to a lesser degree, Lockpicking. However, unlike Rogue poisons, Runeforging a weapon supercedes weapon enchantments. Inscribing Runes onto the Death Knight's weapon also requires a Rune Forge, kind of like a Moonwell for making Mooncloth or a Forge for smelting metals. Currently, Rune Forges can only be found in Ebon Hold, the Death Knights' exclusive clubhouse.
There are currently six runes available for Death Knights to inscribe into their weapons -- Rune of Cinderglacier, Rune of Frostfever, Rune of Lichbane, Rune of Spellbreaking, Rune of Swordbreaking, and Rune of the Fallen Crusader. Some runes mimic existing enchantments, such as Crusader, while others seem very task-specific such as Spellbreaking for taking on caster-type mobs. Death Knights can change the rune on their weapon as often as they like, provided they have access to a Rune Forge. Inscribing weapons with runes also endow the weapon with an animation similar to enchants (Swordbreaking looks like Savagery, for example). Check out the gallery for screenshots of the Rune Forge and the different runes.
Gallery: Death Knight: Runeforging