Birds have special feathers on their tails and wings which play an important role in their flight. These feathers are characterized as contour feathers. The ones on the tail are called rectrices and the ones on the wings are called Tertiaries, secondaries and primaries. The role of feathers is fairly complex and is different in each kind of feather. For complete details with diagrams see the link below:
http://www.paulnoll.com/Oregon/Birds/feather-flight.html

 The wings of birds have a concave shape and air passes above or below the wing as the bird moves forward or as the wind blows. The air that moves above the top of the wing has more distance to cover to get across the wing, thus it speeds up. This causes the pressure to go down since the same amount of air is exerting its pressure over a larger surface area and thus the movement.