Although shin and leg protection dated well before The Romans (Ancient Greece for one), they were eventually incorporated into Roman army use by the late Republic Period. They also carried over into The Imperial Era (depending on the unit). Basically Greaves were curved (shaped) pieces of bronze or brass composite - designed to protect the entirety of the leg, from the ankle to just below the knee cap. In some cases, Greaves could extend a bit higher. Among The Legions, they were unadorned. However, among the Upper Classes (commanders), they were quite elaborate with designs (often made with precious metals). Although in most cases, given the price to make - most commanders would save them for parade or ceremonial wear.