Buckler: This small metal shield is strapped to the forearm, allowing it to be worn and still use the hand. A bow or crossbow can be used without penalty. An off-hand weapon can be used, but a -1 penalty on attack rolls is imposed because of the extra weight on your arm. This penalty stacks with those for fighting with the off hand and, if appropriate, for fighting with two weapons. In any case, if a weapon is used in the off-hand, the character doesn't get the buckler's AC bonus for the rest of the round.
Small Shield: A small shield's light weight lets a character carry other items in that hand (although the character cannot use weapons).
Large Shield: A large shield is too heavy to use the shield hand for anything else.