A sail is driven forwards with the wind 30-45° on the beam.
When air flows along a sail (or an airplane wing) the shape of the sail forces the air flow on leeward side to take a longer path than on the windward side. Therefore the air has to increase its velocity on the leeward side of the sail resulting in a lower pressure than on the windward side. (Bernoulli's principle states than an increase of velocity in a fluid flow gives a pressure decrease.) In effect a sailboat may be sucked through the water due to the low pressure on the leeward side. Conversely a slight increase in the pressure will act on the windward side.
The total sail force may be split into two components, namely lift and drag as shown on the illustration. The lift acts at right angle to the wind and the drag acts in the wind direction. Both lift and drag increase with wind speed but drag decrease increases faster. As a consequence different sailshapes have optimum drift/drag ratios at different wind speeds.
When sailing to windward (beating/close reaching) lift should be maximized. With the wind abaft the beam (broad reaching and running), however, drag works in the right direction and contributes to boat speed.
Extract from Sail and Rig Trim by Ivar Dedekam, Dedekam Design, Norway and distributed by Elvstrøm Sails.