A shield (as opposed to a Buckler) is a large shield that you can either strap to your arm or hold by a grip.
A Shield must be more than 16 inches at its smallest diameter; at its widest diameter, the Shield must reach up no higher than the bottom of your sternum (where your ribs join together) when you set it on the ground, and no more than 36 inches across in width when held in the same fashion.
You can only wield a Shield this size if you have one of the Myrmidon or Abjurer Abilities.
Any attack or projectile that hits a Shield deals no damage. Some Called effects (such as Crush and Repel) may still affect you even if they strike your Shield, but even then you take no damage from the hit.