For centuries, man has sought to protect himself from sharp, pointy, or generally unpleasant objects by shielding himself from attack. Of all manners of defense, shields were the most common and usually the most effective. From the first time a caveman tried to use a large rock on his arm to stop a sabre tooth tiger, the shield has evolved countless times. Be they wood, animal hide, iron, steel, bone, wicker, stone, or everything in between, shields are designed for one purpose. To protect the user from attacks from both close and long range assaults.
In fact, shields are so old that no one really knows which civilization used them first. Even the earliest pictures and languages described shields in some degree. All nations from Rome to India used shields to defend their warriors from harm. Despite their world wide use, there are literally countless kinds of shields. Examples include bucklers, tower shields, guards, hand shields, round shields, triangle shields (these are usually the most common), and even simple hand guards can be described as shields. Shields played an incredible role in Medieval and Ancient Warfare. In the large, open battlefields of Europe and Africa, it was a necessity to put something between you and your opponent and/or arrows. In some armies (Romans being the most prolific), shields were an integral to their military strategy. Legions of foot soldiers would form solid squares of shield armed men to counter and foe. This strategy worked particularly well with spears or pikes.
While the use of shields diminshed quickly with the advent of gunpowder, shields were used all the way up to World War 1! While no modern armies included shields in their arsenal, riot police and SWAT units around the world depend on a strong shield to keep back crazed murders or rioters.
Just like their historical counterparts, shields in video games are almost always used to defend the player from harm. They may come in any shape or form be they high tech energy shields or simple steel and wood.