Plate armor is the heaviest type of armor, heavier than mail, leather or cloth armor, worn by warriors and paladins at or above the 40th level. This ability is taught by class trainers, and costs roughly two gold. Death knights start with the plate armor skill in Wrath of the Lich King.
Although plate does offer the highest amount of armor out of all the armor classes, it also boasts the highest repair bill. A warrior or paladin in plate epics can easily lose 40 gold from dying once.