A Buckler (as opposed to a true Shield) is a shield that is less than or equal to 16 inches (roughly 40.5 centimeters) across at its largest diameter and is held by a grip rather than strapped to the arm.
Any character can wield a Buckler to defend themselves.
Any attack or projectile that hits a Buckler deals no damage. Some Called effects (such as Crush and Repel) may still affect you even if they strike your Buckler, but even then you take no damage from the hit.