A shield is the name used to refer to any object or force used to block something.
The oldest shield known was a device used to block hand weapons and arrows. The shield has been constructed differently over time, even animal hides were used, and the size and weight were largely different as well.
In the Middle Ages, Kite shields were commonly used. The Kite shield is a shield in the form of a kite. Because of the lack of mobility due of the kite shields' weight, kite shields were given up for more mobility and two handed weapons. The Buckler was a shield with a round shape. It was commonly used as a replacement for the heavy Kite shields.
Shields were used even after gunpowder weapons. the shield is still used by police and army forces today. Many shields now in days are usually made from advanced material, as well as electronics. Shields also vary from not only hand helds, but clothing, such as vests and, in a way, boots and gloves.