Photo Source: http://www.unc.edu/~thriveni/sailing/lift.html

Photo Source: http://www.abc-of-sailing.com/info/points-of-sailing.asp

    Motion is generated from the wind force on the sails.  There are two cases that sailboats experience; downwind and upwind.  Downwind is easy to understand, simply using Newton's Third Law there exists an equal but opposite force from the sails.  The force on the sails propel the sailboat forward, at a speed equal to or less than wind speed. 
    The other case is not so easy to grasp and that's going into the wind.  Even more puzzling is how going into the wind is actually the fastest direction to travel.  This can be seen through a vector decomposition of the "lift" force generated in a sail, explained in Bernoulli's Equations.  
    Bernoulli's Equations state that the external force Wext, in this case work is from the wind, is equal to the kinetic and potential energy (Romero). 

 
In the case with sails, you have some air particles moving faster on one side of the sail versus the other.  In order for the two sides of the equation to be equivalent, pressure increases on the slower side that is greater than the faster side.  The difference results in a force on the sail called lift.
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