Paladins are primarily tanks, and very popular ones at that. They have a fair share of Cleric buffs and heals, making them able to buff themselves and party members without having to seek out a cleric. Paladins are not often main tanks in raids, as most raid bosses are immune to stuns. They are however commonly used to cure raid groups and use their group heals to keep the DPS members alive, a very important role indeed.
For soloing, Paladins primarily outlast their enemies. They wear plate armor and this gives them a very high amount of defense. With the ability to heal themselves and a powerful two-handed weapon, Paladins can inflict a good bit of damage while keeping their own health at a safe level. Their stuns reduce the time they are attacked and can interrupt enemies casting heals.