The term adarga simply means "shield" in Spanish, but when speaking of historical shields it has come to be associated with two specific and very different types of shields.
The first of these is the leather adarga. This shield looks like two ovals, their long sides overlapping somewhat. The result looks roughly like the outline of a peach. It was commonly made of wood covered with leather or from two layers of heavy leather laced together. This design was brought to Spain by its Muslim conquerers during the Middle Ages and continued to be used into the seventeenth century. It was light, quick, and primarily intended as a defense against arrows for cavalrymen. This design was taken to the Americas by Spanish Conquistadors, for use against the Native Americans after arrows had ceased to be a significant weapon in Europe. Ironically, some of our best examples of this style of shield were preserved by the Native Americans as trophies of war or medicine charms.
The other type of adarga is a historical anomaly. It consists of a short (~5 feet) long wooden haft with spearpoints at both ends. It is held in the middle, and a small buckler-like shield is attached to the half to protect the hand. This was used by swordsmen as a parrying device in the left hand. The spear can be used to block blows, and the spearpoints can attack the opponent's legs and face. This design is quite similar to a family of related weapons used in India, including the Madu and the Sainte.
This second type of shield's association with Spain is extremely tenuous. There is an example of this shield found in a catalog of the Royal Armory of Spain labelled simply "adarga", believed to have been brought to Spain by Muslim mercenaries. There is no evidence of use of this type of shield by the Spanish themselves. Most likely, the full inscription read something like "Moorish shield" and was simply shortened to "adarga" through laziness. This single example has led to the widespread use of this shield in the SCA.