A buckler is a small shield, used in both heavy and rapier combat. They are usually between 8 and 12 inches in diameter, and primarily constructed of metal and sometimes had leather coverings. They were in common use from at least the 13th century and were common sights for hundreds of years.
While historically most bucklers were round and may or may not include a boss SCA fencers have developed a number of other shapes and innovations. Most common of those adaptations are other shapes such as square, rectanglular and kite shields.
Of the shields per se, it is probably the lightest, and easiest to move about. But, given that it is usually gripped by a handle across its back, it deprives its bearer of the use of the shield hand. That said, it is one of the easiest to attempt to have your opponent do a "nose-butt" on, just by putting it in the way. It also allows for a quite vicious back-hand rim-blow which could break the nose.