A spinnaker is a type of sail designed for sailing off the wind on courses between a reach (wind at 90° to the course) to downwind (course in the same direction as the wind) . It is set forward of the mainsail and balloons out in front of the boat, and is constructed of lightweight fabric, usually nylon, and is often brightly colored. There are two types of spinnakers: symmetric and asymmetric. Symmetric spinnakers are flown with a "guy" and a "sheet," while asymmetric spinnakers are flown with a tackline and a "sheet". The tack attaches to the bow or (often retractable) bowsprit, and the two sheets attach to the clew. The head of the sail is attached to the spinnaker halyard, which is used to raise the sail.

Asymmetric spinnakers are all-purpose downwind sails, while symmetric spinnakers are built for specific downwind apparent wind angles. The best wind-angles for a cruising asymmetric spinnaker are between 80 and 120 degrees off the apparent wind, that is, from just forward of the beam to a broad reach.

To use a spinnaker, it must be rigged and hoisted properly. There are many resources available online, including step-by-step guides and videos, that can help sailors learn how to use a spinnaker effectively.