What is difference between keratoconus and "common" astigmatism and what do the numbers mean?

Astigmatism is a common condition where the curvature of one or more of the optical surfaces of the eye (the cornea and lens surfaces) are more "round" in one direction than the other. In "regular" astigmatism the maximum and minimum powers are aligned at 90 degrees to each other while in "irregular" astigmatism they do not align. An egg is a good example of a surface with "regular astigmatism". Keratoconus is a degenerative condition where the cornea thins in affected areas. This can lead to astigmatism - often regular at first but becoming increasingly irregular as the disease progresses.