Webcreeper

It is a physical effect used to explain how lift (upward force) can be generated by moving air acting on a wing:

http://www.mountwashington.org/education/center/arcade/bernoulli/index.html

http://www.rmsc.org/museum/kidsclub/experiments/airplay/airplay1b.htm

More technical:
http://en.wikipedia.org/wiki/Lift_(force)

But there are different views:

http://user.uni-frankfurt.de/~weltner/Flight/PHYSIC4.htm