Shields are special weapons that are accessible to all classes except Vanguard. They give their user the capacity to block attacks without too much attention to timing, the ability to easily close the gap against ranged weapons, and subsitutes the user's kick for a less-vulnerable shield bash. The only exception to this is the Pavise shield, which cannot be used in a traditional manner.
Even when unused on the players back, any shield will always protect the user from projectiles so long as the shield's shape gives the proper coverage, making it useful when running away from Archers or other ranged weapon users. However, shields will only guard against melee attacks when the user holds it to block properly, so a melee attack to your shielded back will still hurt as if you had no shield at all.
It is occasionally possible to attack over or behind a shield by attacking with a long weapon. See the Strategy page for details.