The paladin started out as a D&D class, as a holy warrior of lawful alignment. Paladins are still usually lawful, and are portrayed as having a strong sense of honor and chivalry. Their most common special abilities are the Lay Hands healing spell, the ability to Detect Evil, and a selection of Cleric spells of a lower level than a corresponding Cleric. With regard to combat abilities, a Paladin is usually slightly inferior to a pureclass Fighter but does not necessarily become obsolete as it fills the role of the holy fighter which gains an advantage depending on the faith they are fighting for.
Typically a paladin is your heavy fighter but with the twist of witch-hunter and undead-hunter mixed into the lore. They will use holy water, cleansed and blessed weapons and armour, spells which destroy evil and radiate an aura of good around them. A paladin will usually be a more weapon based warrior than a spellcaster too, though they are capable of casting if generally speaking, though in-game perceptions of a paladin may vary from game to game.
In EverQuest, paladins are the knights of all that is good and just. They are given the duty of defending against darkness and evil at all costs. Paladins are essentially a hybrid of the Warrior and Cleric classes. While nowhere near the healing ability of a cleric, Paladins do carry the powerful ability of Lay on Hands, which instantly fully heals himself/herself or a friend, but can only be used once per day. Much like Clerics, Paladins excel against undead. They do not have the ability to dual wield, but can heavy damage with an assortment of two-handed weapons or protect themselves with a weapon and shield combination. Paladins can wear any of the four armor types, but most often choose plate.
Paladins in the Warcraft lore worship the Holy Light, which is not so much a deity or god but a philosophy, and through that worship and training they are able to use the Light to their advantage.
The Paladin class in World of Warcraft is a Hybrid class that can be used as a healer, damage dealer, or a tank. They are able to wear the heaviest of armors as well as a shield making them one of, if not the most, defensive class in the game. The 3 talent trees in the game are all related to each role the class can do:
Holy - The Paladin focuses on healing spells and abilities. Using spells like Holy Light and Flash of Light as their main healing spells, but can also use spells like Beacon of Light to double its effect or Divine Illumination, which will lower the mana cost of these spells.
Protection - The Paladin focuses on tanking, which involves grabbing the attention of the enemy and taking the damage while others destroy it. They use their shield for many abilities, such as Holy Shield to raise defense while doing holy damage back to the enemy or just plain throwing the shield at them with Avenger's Shield.
Retribution - The Paladin focuses on dealing damage to his enemies. Putting away the shield for a 2-handed weapon, the Paladin will use devastating abilities like Divine Storm, which will create a storm of hammers around the Paladin hitting everything within or Crusader Strike, which will empower his weapon with holy energy and then immediately strike his opponent.
All Paladins have some shared abilities as well .They have auras and blessings that will help out with the party or raid giving them a buff depending on the spell. They also have Divine Shield which will make them immune to all damage for a short period of time. During this time the Paladin can heal themselves, do some extra damage, or in case they are in real trouble, use their hearthstone and "bubblehearth" to get out of there.