Here's a simple explanation of the types of balsa grain and the best use for each type of grain. From Model Aviation Magazine, for more detailed information: Click Here
The density PDF is below for your convenience.

Builders should take note of the different balsa grains. Each grain type has different characteristics and should be used in different applications. Notice that the C-grain balsa sheet has a mottled appearance and distinct checkerboard pattern. It is very stiff across the sheet and splits easily. It will not easily wrap around curved surfaces without cracking. C-grain is used for sheet balsa wings and tails, flat fuselage sides, wing ribs, formers, and trailing edges. C-grain is usually hard to find in local hobby shops. I mainly use it for sheet tail surfaces. B-grain grain lines are shorter than A-grain and it is less stiff than C-grain. If you look at the narrow edge of the balsa sheet, B-grain will look the same as it does on the wide, flat side. B-grain can go around soft curves without cracking. This grain is used for flat fuselage sides, wing ribs, formers, planking gradual curves, and wing leading edge sheeting. A-grain has long fibers that show up as long grain lines. If you look at the narrow edge of the balsa sheet, A-grain will have a checkerboard appearance. This is the most flexible of all grains, and it is quite easy to wrap around curved surfaces. A-grain is typically used for sheet covering rounded fuselages and wing leading edges, planking fuselages, forming tubes, strong flexible spars, and hand launch glider fuselages. By Al and Rod Clark