A Paladin is an individual with a mastery in the melee combat of a warrior, and the power of the Holy Light of a priest. Paladins are considered "Warriors of the Holy Light", combating for all that is good and against those that are evil, such as the undead, and the Burning Legion.
The first generation of paladins had spawned the Order of the Silver Hand under the Leadership of Uther the Lightbringer during the second war as a way to counter the orcish death knights. Paladins were also quite useful in combating the undead, as paladins were immune to the plague.
All paladins uphold the tenets of the Holy Light. Though, there are some (such as the Blood Knights, or Twilight Paladins) who do not supplicate to the Holy Light. For example, a Blood Knight siphons off the powers of a Naaru, this Naaru being M'uru. A Twilight Paladin supplicated to the Old Gods to instill them with the powers of the Old gods, or Twilight magic.