The mainsail is the primary sail used to operate a vessel. It is connected to the boat via a vertical support (mast) and horizontal support (boom). This sail not only provides the main source of speed and power, but is also used to steer the boat. Unlike most others types of sails, the mainsail is kept in virtually the same configuration throughout a sailing trip. In extremely high winds, the mainsail may be folded down to decrease its surface area in a process known as "reefing." Another alternative to keep the boat under control in high winds is to lower the mainsail and replace it with a much smaller "trysail."
A headsail is used for sailing against the direction of the wind, known to sailors as "upwind." It connects to the mast of the boat, with one corner tacked to the forefront of the vessel. There are two basic types of headsails used in sailing vessels. A "jib" is a small headsail used in high winds, while a "genoa" is larger and is used when wind levels are low. The genoa is so large that it overlaps the mainsail slightly, while the jib does not go past the mast.
Spinnaker (shown in picture at top)
The spinnaker is the largest sail on a boat, and it comes in a wide variety of shapes and sizes. It is primarily used for sailing directly away from the direction of the wind, or "downwind." The spinnaker creates a balloon-like shape that extends out towards the front of the boat, and it is highly effectively when used to sail downwind. It allows the boat to travel very quickly, and users often describe the use of the spinnaker as "running" or "flying." Symmetrically shaped spinnakers work best when the wind is directly behind the boat, while asymmetrical versions are used when the wind is coming from an angle.