Angle of attack, officially known as the angle between the relative wind and the chordline of the wing.
With sufficient angle of attack you can make pretty much anything fly
see this video:
http://www.youtube.com/watch?v=C_3vLmm5tC8
With inverted flight the wing is upside down, therefore it needs a higher angle of attack then in normal flight.
Watching a plane come by in inverted flight, you will see the nose is fairly high up, this is to create sufficient lift by flying with a high angle of attack.
A aerobatic airplane specifically made for the job can have what is a called a symmetrical wing profile, the top and bottom are the same shape. It therefore flies about as good inverted as right side up.
Normal wing;
