A buckler is a small shield, used in both war with heavier single handed swords and civilian duelling with swords such as the rapier. 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.
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.
Swords and bucklers were, for a time, very much a fashion accessory for well-bred young men. The buckler, when not in use, was hung from the hilt of the sword, and the clatter of metal buckler on the metal sword hilt was a distinctive sound warning of the approach of a "swashbuckler", that is, an armed gentleman inclined to duel.
Bucklers in the SCA
While historically most bucklers were round, SCA fencers have developed a number of other shapes and innovations. Most common of those adaptations are other shapes such as square, rectangular and kite shields.
See The Sword and Buckler Tradition for a multi-page article with many illustrations from period sources.