Reefing

Reefing is the means of reducing the area of a sail, usually by folding or rolling one edge of the canvas in on itself. The converse operation, removing the reef, is called "shaking it out." Reefing allows the carrying of partial sail in strong winds, and is the primary safety precaution in rough weather. Reefing improves vessel stability and minimizes the risk of damage to the sail or other gear. Proper skills in and equipment for reefing are crucial to averting the dangers of capsizing or broaching in heavy weather.
Conventional reefing.
Sails may have built-in alternative attachment points that allow their area to be reduced. In a mainsail, pairs of grommets, called reefing tacks, reefing clews, or reefing cringles may be installed in the sail; a cruising boat will typically have two to three pairs. Pulling these points down to the boom forms a new tack and clew, reducing the sail's area. Using the pair of grommets closest to the boom is called a single reef, using the next pair is called a double reef, and so on. A sail may have "reef points," small holes in the sail between the grommets. These are used to secure the excess fabric of the sail after reefing to minimize flogging and improve visibility from the cockpit. 
To reef the sail of a Bermuda rigged sloop:
Roller reefing.
Roller reefing rolls or wraps the sail around a wire, foil, or spar to reduce its exposure to the wind. In mainsail furling systems the sail is either wrapped around the boom by a mechanism in the gooseneck or hardware inside the boom winds it around a rotating foil.
Furling systems controlled with lines led to the cockpit allow reefing without crew having to go on deck in heavy weather. Roller reefing also allows more variable sail area than conventional or jiffy reefing. Countering these advantages are the furled sail possibly not having an optimal shape and sail repair or replacement being more difficult.
Slab reefing.
Slab reefing, also called "jiffy" or single line reefing, is an alternative to conventional or roller reefing where the sail is folded in sections, or slabs, along the boom. One or two reefing lines reeved through the sail's luff and leach reef cringles create a new tack and clew for the sail by pulling those points tight to the boom. These can be led back to the cockpit to allow crew members to reef without going on deck in heavy weather. Intermediate reef cringles are not used.
The equipment for jiffy reefing is sometimes integrated with Dutchman flaking, a patented furling technology that helps to flake the sail on alternate sides of the boom. The Dutchman system keeps the topping lift fairly taut when reefing. Two or more "flaking lines" run down vertically to the boom, and pass through a number of matching cringles on the sail, so that the latter may be flaked in the manner of a Venetian blind. Simple in concept, the system can be subject to frictional sticking, whereupon the flaking lines can actually hinder reefing once the wind gets up.