Typpically the term is used for reefing a sail, but can refer to changing to a smaller sail. Reefing is accomplished by lowering the sail down the mast or stay via halyard, and reefing or making up the foot of the sail with ties so it remains aerodynamic and isn't messy.
"Reefing the sails" reduces the area of sail.
Making the sail area smaller is called reefing.
Roller reefing is simply not deploying the full sail. Deploy the sail until flogging occurs and then reef in until it stops. With a little practice this can be achieved very quickly
For those using sails, it means reducing the area of exposed sail to add safety and stability to the boat. http://en.wikipedia.org/wiki/Reefing
"Set the yards" means arrange the sails in preparation for leaving the docks/shore/what have you: "make sail" means to actually go out into the ocean/lake. With regards to older sailing vessels (tall ships, square riggers), "man the yards" meant to send men up the mast and out along the yardarm of a square-rigger in preparation to making sail. "Making sail" meant either unfurling the sail from the yardarm, or un-reefing existing sail so as to add more sail.
Reefing is the process by which a sailboat reduces it's sail during windy conditions. Usually, the sail is either gathered in and tied to the boom, or it can be rolled in on sailboats that are so equipped. In particularly windy conditions, it may be necessary to use a double reef to further reduce sail. I'm not sure about "second reefs," but I would assume that it's the same as a double reef.
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Depends on the boat and whatever you are looking for. I like 10 to 15 knot range in my 30 footer. It is 6500 lbs and have no problem reaching hull speed with these winds. By reefing you can extend your comfort for higher winds.(less heeling which tends to require more force to control tiller or wheel.)
Officers of the lowest Naval rank (Midshipmen) had the duty of reefing down the sails. They were referred to as "Reefers".
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There are sail planes, sail boats, sail cars, and with a spaceship you could sail to the stars.