The Paladin is a holy warrior. As a game archetype it originates in Dungeons & Dragons, which really couldn't decide if he was supposed to be a medieval religious fanatic or a modern cop. Fortunately, we are discussing roguelike games and can ignore such role-playing considerations.
A Paladin typically is good in melee, but not necessarily in ranged combat. He has special holy powers that help him to smite demon and the undead. He may also have spell casting abilities and will have a close relationship to his god, if religious elements are included in the game. Stereotypical Paladins will also wear a lot of armor.