Actually, it does both depending on how you use them. The sails can move so a person sailing a boat can catch the wind by how they move the sail or they can also stop the boat by the movement…
Fx Sails offers quality and affordable sail boat sails. You can find them at: www.fxsails.com/. In your town, there should be marinas near where you launch your boat that will offer sail repair.
A sailing boat can not sail directly upwind, sail at about 45 degrees to the wind and tack (turn the boat through the wind) to the other side of the wind and continue in this zig-zag manner up wind.