The Death Knight is the first Hero Class in World of Warcraft. All races can be Death Knights.
The criteria for creating a Death Knight is the existence of a level 55+ character on the player's account. Once that criteria is met, the player can create one Death Knight per realm on any realm where they have a level 55+ character. This is a separate character from your original one and can be created as soon as one of the existing characters in your account reaches level 55. After making a Death Knight, you will not lose the access to any of your existing characters.
In general, the death knight can be considered a hybrid class that combines damage dealing and tanking. They will wear plate armor, and be able to dual wield or use two-handed blade weapons and maces. Like druids, they will tank without shields. Their tanking mechanics will most likely be high armor as indicated by Frost Presence, and by having a high chance to parry.