Oyoroi (or o-yoroi) is armor (yoroi) worn by samurai and generals and the like. It is very distinctive, and is the kind of armor that most people probably think of when they imagine a samurai mounted on a horse.
Oyoroi started to appear in the 10th century, and is a big, boxy and poorly fitting kind of armor used by mounted archers. It is also fairly heavy, generally weighing about 60 pounds. It was also expensive, and only for the wealthy.
The armor is constructed from small scales of leather or metal (preferably iron), laced into a plate, and then lacquered. These laquered plates are then laced together to form the armor. In less vital areas, the plates themselves were leather, to reduce weight. The whole thing is topped by a helmet (kabuto).
Footmen would wear a less restrictive form of armor, like domaru or haramaki.