The bronze cuirass, or breastplate was the armor of choice for the Spartan Hoplite. It was also used by Alexander the Great and Hannibal in the show, and historically by other warrior cultures like the Celt's, Romans, and Spartans. It was made out of a single sheet of bronze as was the case for most Greek armor.
A Bronze Cuirass offered excellent protection from most weapons it would come up against in it's day. Swords and spears of the day lacked the penetrating power needed to harm the wearer. It was sometimes shaped in the form of a very well developed body as if to show of the muscular physique of the Spartan warrior, this was a more specific type of armor called Musculata, which was design to intimidate enemy.
While offering sturdy protection the Bronze Cuirass was heavy and expensive for use in regular foot soldiers. These short coming caused the Roman's to drop it favor for other armor types such as Lorica hamata and Lorica segmentata.
Multiple warriors have used the Cuirass in the show, the Spartan was the first. In the second and third season's Alexander the Great and Hannibal were both given cuirass. The effectiveness of the cuirass's ability to stop bladed weapon is show multiple times, both the Ninja's Kusarigama and Genghis Khan's saber failed to cause injury through the curiass.
However, in the show most pieces of armor claimed to be bronze, appear to look more like brass. This is a common practice in modern reproductions of bronze armor, brass being much cheaper than bronze.