Knight

Knights are traditionally horse-mounted warriors of the Alliance and the kingdoms of humanity. They are usually humans, although other factions have knights as well such as the elves. Aside from champions, the knight is the warrior elite of humanity, especially of the Alliance, the best of the normal troops thus among the most feared. They typically form ranks and charge, which cause devastating effect on the battlefield. This proud young man carries himself as if he were the lord of the battlefield. Resplendent in his (probably hereditary) armor, he spends much of his time testing the balance of his lance and speaking with his horse. A knight's most favored weapons are the sword and mace. When one becomes a knight, he is given the title "Sir" in front of his name.

Note that from a Lore perspective, it is entirely feasible for Forsaken and Undead to technically be Knights, although these are often refered to as "Dark Knights", and seen as a more anti-Paladin class.

More about Knights can be read at:

http://www.wowwiki.com/Knight