Zerozero got it in a nutshell.......as he says, it's explained by Newton's Third Law.
A while ago I stumbled across this NASA web-site (not rocket science in this case, honestly - its actually a Beginner's Guide To Aeronautics) which gives some superb explanations to this and probably every other aeronautics conundrum you can think of. Give it a try.......
http://www.grc.nasa.gov/WWW/K-12/airplane/
WUT