Since both the sail and the keel are traveling through a fluid, air and water respectively, they both experience a lift force as shown in A.  In the diagram B, the forces are shown.  Decomposing in the x-direction, the majority of the forces cancel each other.  The resultant force (net force) becomes the y-direction or forward-direction.  Since wind will continue to flow across the sails even as the sailboat reaches wind speed, sailing into the wind therefore allows greater speeds than downwind (downwind is limited by wind speed) (Anderson).