The shield has two major variations. The normal shield which is used in conjunction with a weapon and the tiger shields which is two shields which have a deep-V cut into the top and the inside sharpened. Many cultures have the shield with edges concept but they weren't as popular.
The normal shield tends to be considered round but any shape desired could be used. The purpose of the shield was to trap the opponent and hide the other weapon and footwork. Many people use the shield mainly to block but that only works until the shield breaks of the opponent finds a way around this defense. Cultures which used the shield to block always progressed to a larger shield. Anyway the shield could also be used to generate more power in the weapon being used. When the shield is trained this way the first levels are often teaching the student how not to lose the shield. Opponents try and take the shield away or use the shield against the wielder. It makes a nice handle for joint locks and throws. When the user can dependably keep this from happening then they are allowed to use a weapon with their shield.
The Tiger shields where used by people who tended to charge into battle. They tended to be stronger than average. The shields were basically used to block everything on the charge in and the practitioner would use short punches to bring the sharp edges in contact with a joint, usually the throat. With two shields the wielder could effectivly stop any number of weapons and often kicks would be used to break up the formation of the opponents. The techniques are few, simply, and brutal. Obviously they were heavy and perfect for the battle field. Unless you were a waitress there isn't a whole lot of non-combat use for the tiger shields and finding people who trainind in them is difficult. I only know of my teacher and Wing Lam. I believe Wing Lam also sells a tape with a shield form and they sell the shields also. The weapon is from the Hung styles so there may be many people with this training.