Well, it’s a triangle so there are 3 points. The top point is called the head. The head is attached to the halyard (the line – remember we have lines never ropes) which raises the sail.
Below the head we have the tack and the clew. The tack is attached to a fixed point on the boat close to the mast in the case of a mainsail, or a forestay in the case of a headsail. The clew is aft (at the “back end”) of the sail (furthest from the wind).
The lowest edge of a sail is called the foot. The foot is between the clew and the tack. (or in the case of a spinnaker between 2 clews – spinnakers to be explained later).
The leading edge of the sail (closest to the wind), also known as the forward edge is called the luff.
The aft edge (furthest from the wind) is called the leech.
Sails are not always a perfect triangle. There is an arc of extra material on the leech of a sail (most common on mainsails). This is the roach. The roach is supported by battens because without a batten the roach would flap too much.
“Reefing” a sail means to shorten a sail. You shorten a sail by tying it into a smaller shape. A reef point is a reinforced hole in the sail that you can thread a small length of line through and tie the sail down over the boom.
Battens are long thin strips of usually fiberglass that are used to support the roach of a sail. They go into specially designed batten pockets in the sail.
Telltales are thin, lightweight pieces of fabric or yarn that are used to properly trim (adjust) the sales for the best performance. Telltales are in pairs one of each side of the sail. Optimally the telltales both fly straight out, if one is spiralling rather than streaming it is indicating that the sail has incorrect air flow on that side. To fix this the sail needs to move towards the opposite side.