Sails are mainly used with boats.  Sails are curved and it is designed in such a way that it creates a region of high pressure and low pressure around it. Sails can be compared with the wings of any airplane. On any wing there is a creation of high pressure on the bottom surface and low pressure on the upper surface. So, the upper portion of the wing is at a lower pressure and the bottom surface is at high pressure. The wing is designed in such a way that the flow of air over the top surface is at a higher speed and the flow of air at the bottom surface is at lower speed, this creates a region of lower pressure on the top surface and a region of higher pressure on the bottom surface, resulting in a net upward force of lift. 

Sail on a boat

Similarly on the sail there is a lower pressure on the front side and a higher pressure on the back side due to its curved design.  The flow of air on its front side is at a higher speed creating a low pressure region and the flow of air behind the sail is at a lower speed creating a high pressure region. This difference in pressure creates a net forward force like the lift of the aircraft. 

This is how a sail works. So, we can say that the working of the sail in the  boat  is an analogy to the working of the wing on an aircraft.