Definition for light, medium and heavy flour?
The blog sourd'oh referenced is the best I've seen up to this point:
http://www.myrealfoodlife.com/understanding-gluten-free-flours/

Basically, My Real Food Life breaks it down into the following categories:


Heavy: Give structure and binding, have lower rising capacity.
Medium: Lighten heavy flours, even out stronger tastes, soften the
crumb and texture.
Light: Needed for binding, adding lightness, and sometimes
crispiness.  Neutralize the taste of stronger flours.


If you're looking to use a wheat-based flour, this seems to correlate to the flour's milling and the type of wheat used (hard seems to correlate with heavy and soft seems to correlate with light).  


Heavy:  Whole wheat flour, bread flour
Medium: All purpose flour
Light: Cake flour


For additional information on the composition of wheat-based flours, see the following Wikipedia article:
http://en.wikipedia.org/wiki/Wheat_flour