The Death Knight is a hero class which was introduced in the Wrath of The Lich King expansion pack. In order to create one, you must have a character who is at least level 55. Upon reaching level 55, the Death Knight class becomes unlocked and you can make one on any realm, not just the same realm as your level 55 character.
The original concept of a Death Knight arised from Warcraft II: Tides of Darkness, which featured an undead spell caster unit. Blizzard later introduced the Death Knight as a hero in Warcraft III: Reign of Chaos, and this unit was much more like the Death Knight which we see today in WoW.
Death Knight talents are split into three talent trees, each specializing in different things.
Although all trees are capable of tanking and putting out competitive DPS, some are better than others at certain raid roles. For example, at the moment, Unholy generally does the highest DPS and Frost is the best for PvPing and Tanking. This leaves Blood, which are makes decent tanks and average DPS. It is generally agreed that Blood is the best leveling spec due to its self healing capabilities.