You might say that paladins are the guardians at the gates of hell -- they fight evil wherever it penetrates into their world and they take the fight to the evil's source in the hope of quenching it forever. Although they focus on guarding their people from undead and demonic forces on the rise, paladins actually stand against evil everywhere, including the evil in their own hearts.
Being a paladin means that you have a relationship of some sort with the Holy Light, that mysterious force of goodness and faith that flows to some degree within all living beings with positive intentions. Most paladins (and many priests) believe that when you do something that you believe to be good, the power of the Light increases in you and your connection to the rest of creation is strengthened, whereas doing something evil (such as acts of greed, despair, or vengeance) will darken the universe and weaken your connection to it. Whether this belief system is a religion or a philosophy is open to interpretation, and seems to depend in some part upon which race you are.
There are three sorts of paladins in World of Warcraft, aligned with the humans, the draenei, and the blood elves. All of these share certain similarities, but each has its own differences as well.