Sails

I will discuss only triangular sails here, because they are by far the most commonly used sails nowadays. If you're interested in other rigs please go right here, and here.

Types of sails
Mainsail	This is the large sail situated behind the main mast (the one attached to the mast and the boom). It is tensioned by the boom. The leech can be stiffened by battens inserted into the batten pockets. In very stormy weather a sail called a triysail can by used instead of a main.
Foresail/headsail	A foresail is the sail (such as a jib) located immediately in front of the main mast. It is attached to the forestay. A headsail is any other sail located in front of the main mast. Headsails can be divided into three types of jibs (jib 1, 2 and a storm jib)... and two types of genoas (genoa 1 and 2). Genoas are larger than jibs, with genoa 1 being the largest. Storm jib is the smallest headsail.
Spinnaker	This is the largest sail on a boat. It is a very light headsail used when sailing downwind (running), or on a broad reach (much more difficult). Like in the main, the top of the spinnaker is the head, and the bottom is the foot. The luff is the windward edge, and the leech is the leeward edge.

The very first sails were constructed from such materials as skins, flax, cotton, bamboo, or coconut fibers. They were very prone to stretching and rotting, however, and now we use synthetic yarns instead of natural ones. Dacron and Terylene are most often used for mainsails and headsails, and nylon for spinnakers. Mylar (which is a non-woven sheet of plastic) with a woven backing of Kevlar is one of the latest materials used for sails. However, these are very short-lived, especially when you always want to bring the best out of your sails, as for example in racing.

Different weights of cloth are used for different speeds of wind. Gossamer cloths (extremely light, used for spinnakers) are used for lightest winds. Spinnakers have various designs, which are determined by the design of the cloth panels: vertical (radial), horizontal, star, and tri-radial. The tri-radial cut is a combination of the remaining three, and is the most universal of them, with good handling and power characteristics. The radial head cut is a lighter weather sail, the star cut is a good strong wind reaching sail, and the horizontal cut is also a good light wind sail.

Horizontal cut	Radial head cut	Tri-radial cut