Knights are traditionally horse-mounted warriors of the Alliance. They are usually humans or high elves, although other factions have knights as well. Knights appeared as units on the human or Alliance side in Warcraft I, Warcraft II, and Warcraft III.
Paladins are also called knights, though they differ from their brothers in arms.
Knights of the Kingdom of Azeroth
During the First War the Kingdom of Azeroth (later known as Stormwind) was served by an order of knights known as the Brotherhood of the Horse. They formed the fighting elite in Azeroth's Majesty’s service.
Using the difficult skills required for mounted combat, these warriors wield huge maces in order to crush their opponent’s skulls. Their warhorses make them fast and deadly, as they can maneuver much more quickly than an unmounted foe. Knights are well respected throughout the realm, and the tales of their fighting prowess are known far and wide.
Knights of the Kingdom of Lordaeron
The Knights of Lordaeron represent the fiercest fighting force in the armies of the Alliance. Protected by suits of heavy armor, they carry mighty warhammers with which to crush all who would threaten the freedom of their lands. Astride great warhorses, these honorable and just warriors serve as a symbol of order to the peoples of Lordaeron in these dark and chaotic times. Having learned of the fate the Knights of Azeroth suffered after the First War, they have sworn to both avenge their fallen brethren and free their homelands from the grip of the Orcish Hordes.
Though the hearty knights of Azeroth were destroyed during the First War, the shining knights of Lordaeron still continue to serve amongst the warriors of the Alliance. Wading into combat astride their noble warhorses, the knights are renowned for cutting bloody swathes through enemy ranks. The knights' speed and mobility mark them as some of the most versatile warriors of the Alliance.
Dark knights were a faction created by the Forsaken, but later replaced with the more rogue-like Lightslayers. Lord Garithos, short lived commander of the Alliance renmants in Lordaeron was also called Dark Knight, though the meaning of this title is not certain.
Knights in World of Warcraft
In World of Warcraft, knights are employed by several factions, including the Death Knights and Necro Knights of the Scourge, and knights employed by the Scarlet Crusade. The blood elves have formed a paladin order called the Blood Knights.
Knight was also a rank title given to people for PVP prior to the revamping of the Honor System.