sails or skins or coverings of a fighter kite all mean the same thing. it is the material used to cover the kite frame.
the most common material for a kite sail is paper. almost any paper can be used, but the lighter the weight and stronger and more durable the better it is for optimum flying performance. the next most popular sail materials are various plastic films, such as polypropylene and mylar-like films. often gift wrapping plastic films are used.