How do sails on a sailboat work

you put the sail up and the sail will catch the wind and you will go

they catch wind currents to propel the boat forward

Sails work pretty much like the wings on an airplane. Their shape is very similar to a wing, at least in cross-section. As the wind flows over a sail, the sail produces 'lift' like a wing, but a sail is positioned so that the force being generated, moves the boat forward instead of lifting it out of the water.

There r types of sails that only work downwind, & these do not work like a wing, but work by catching as much air in them as possible & creating drag, which 'drags' the boat along with the wind.

Here is a link to a more technical explanation:

http://physclips.unsw.edu.au/jw/sailing.html

Threesheave is close.

Only Downwind sails.as in a Spinnaker, capture the wind.

Upwind sails.the front & back sails most non-sailers picture in their mind do not capture wind. They actually direct wind. To say they create Lift is missleading as well.
What they DO is actually create positive pressure on the windward side of the sail when properly trimmed & negative pressure on the back or leeward side of the sail. The sails pull the boat in the direction of the negative pressure. The combined forces of negative pressure behind the sails & negative pressure behind the boat is underwater foils(keel & rudder) creates a resulting lift or windward tendancy in the boat.
The same is true for planes.they r not actually lifted.they r pulled up by negative pressure above the wings.

Some study would be Newton is 3 laws & Bernouli is
Principle.
>>update, for David (next post)
Read what you're saying..the low pressure area is on the leeward side of the sail.the long curve.in relation to the wind direction this is the back side of the sail. Not to be confused with leading edge(luff) or trailing/back edge(leach).
And yes, a percentage of a boats movement will always be sideways.leeway.

A couple of the above posts have the right idea, but have a few things turned around.

When sailing downwind, the sails r let out & create a greater surface area to be pushed. This area can be increased by flying a spinnaker in front of the boat. (However, a headsail & or mainsail can also act to just push the boat, even when sailing downwind)

When sailing up wind, sails r sheeted in & create a foil or wing as mentioned above. It is important to have a proper trim maximizing the sail area having a smooth wind flow across it. Since there is a greater distance on the outside of the curve (Most likely up wind,when sailing to windward), there will be a low pressure area on the front side of the sail (not back). This will cause the boat to be sucked forward or upwind. (In otherwords, sailing upwind sucks.) Most sloop rigged boats can do this to about 45 degrees to wind before they loose airflow on the front side of the sail.

This pull is not just straght forward but to leeward as well, but the leward forces r mostly canceled by the boat design, most notably the centerboard or keel. However, the boat is still pushed a bit downwind from where it is pointed & this loss is known as leeway. Many boats r sloop rigged & will have a headsail & mainsail both acting as foils. The headsail, not only acts as a foil, but drives more wind across the front of the mainsail, thus increasing the pressure gradient on that sail.



Update

Note to Dave M. - yes l agree the negative (actually low)er, not negative) pressure is not on the windward side of the sail. The point l was trying to make is that when sailing upwind, there is a low pressure in front of the sail, not as u said: (and l quote) ''negative pressure on the back'': The negative pressure or lower pressure is in fact on the front side. This is what moves the boat forward & somewhat upwind or to windward. The ''suction'' is towards a low pressure on the front of the sail which moves the boat forward. While this low pressure is technically on the leward side of the sail, it is on the front side, not back, & will pull the boat forward, somewhat upwind when sailing upwind - for example a close reach or close hauled. The lower pressure (negative as u put it) is on the front of the sail, not on the back as u stated. If the low pressure were on the back side of the sail, a sailboat would move backward.