From the balsa facts page from Sig: here is the link to some facinating info if you like that kind of stuff!: http://www.mat.uc.pt/~pedro/ncientif...s/techbal.html
A-GRAIN sheet balsa has long fibers that show up as long grain lines. It is very flexible across the sheet and bends around curves easily. Also warps easily. Sometimes called "tangent cut."

C-GRAIN sheet balsa has a beautiful mottled appearance. It is very stiff across the sheet and spits easily. But when used properly, it helps to build the lightest, strongest models. Most warp resistant type. Sometimes called "quarter grain."

B-GRAIN sheet balsa has some of the qualities of both type A and type C. Grain lines are shorter than type A, and it feels stiffer across the sheet. It is a general puropse sheet and can be used for many jobs. Sometimes called "random cut."

I have a sample of C grain on the left and A grain on the right for you to see

Robert