Death Knights can imbue their weapons with special rune enchants. This is their unique class ability. During their starting quests they will learn the ability "Runeforging". While this ability is limited to one skill point, the Death Knight is able to train any rune enchant within his career.

A rune enchant requires the Death Knight to use a runeforge. You can find one of those in the starting area, where you can go any time with the help of the death gate spell. Another rune forge can be found in shadow hold in Icecrown. To access this runeforge you will have to complete a short quest line to free the shadow hold from the hands of the scourge. Only then will you have access to the runeforge and the local merchants, including the quartermaster of the Ebon Blade.

There is a total of eight different rune enchants. Two of those are only usable on one-handed weapons but there are similar versions for two handed weapons. Naturally the enchantments for damage dealing and tanking are different. The Swordbreaker Rune is the best tanking enchant as it increases your chance to parry an incoming attack. The strongest offensive rune is the "Rune of the Fallen Crusader", which is similar to the old "Crusader" enchant. This runes provides a small heal and greatly increases the user's strength. Worth of special mentioning is the rune of cinderglacier, which makes the target more susceptible to Frost damage. This used to increase the damage of frost and frostifre mages, but this has been patched in version 3.1 to only affect the Death Knight's own damage.
Unfortunately rune enchants can not be used together with ordinary weapon enchants. Since they are stronger most of the time this is a small price to pay. Additionally a Death Knight does not have to pay anything for a rune enchant, unlike the normal weapon enchants, which are not so cheap.