A windsurfing sail is made of monofilm (clear polyester film), dacron (woven polyester) and mylar. Sensitive parts are reinforced with kevlar mesh.
Two designs of a sail are predominant: camber induced and rotational. Cambered sails have 1-5 camber inducers, plastic devices at the ends of battens which cup against the mast. They help to hold a rigid aerofoil shape in the sail, better for speed and stability, but at the cost of maneuverability and generally how light and easy to use the sail feels. The trend is that racier sails have camber inducers while wave sails and most recreational sails do not. The rigidity of the sail is also determined by a number of battens.
Beginners' sails often do not have battens, so they are lighter and easier to use in light winds. However, as the sailor improves, a battened sail will provide greater stability in stronger winds.
The windsurfer in the foreground is using a camber induced sail and is fully planing using the footstraps, while the other is using a rotational sail and is not planing.
Rotational sails have battens which protrude beyond the back aspect of the mast. They have to flip to the other side of the mast when tacking or jibing, hence the rotation in the name. Rotational sails have aerofoil shape on the leeward side only when filled with wind. They can be absolutely flat and depowered when sheeted out.
In comparison with cambered sails, rotational designs offer less power and stability when sailing straight, but are easier to handle when maneuvering. Also, rotational sails are much easier to rig.
The leading edge of a sail is called the luff. The mast is in the luff tube. The rear edge is called the leech. The front bottom corner of the sail, where the mast foot protrudes, is called the tack, and the rear corner, to which the boom is attached, is called the clew. The bottom edge, between the clew and the tack, is called the foot.