A shield is any broad, durable held item that can be used to protect the wielder from incoming attacks. The Defense skill concerns your ability to use shields effectively.
Shields are not impenetrable. Like all items, they have a durability and condition score and will break when damaged too heavily. Shields also have a Shield Rating (SR), which is the amount of damage a shield can block in a single blow without taking damage.
Whenever a shield absorbs an amount of damage greater than its shield rating in a single hit, the GM makes a Durability check against the shield. If the check fails, the shield's condition deteriorates by 1.
If a shield ever takes a hit greater than twice its shield rating, that attack penetrates the shield. The wielder takes half of the damage of the attack, and the shield's condition automatically deteriorates by 3.
When shielding yourself with an object that wasn't meant to be used for such purposes, the GM determines the improvised shield's rating and current condition. If the object doesn't have a handhold or other way for you to easily keep hold of it, you take a -5 penalty to Defense checks involving the shield.