I've recently started making rocket gliders again.  I must be good at
it because I've lost every glider I've made so far.  I was discussing
boost gliders with long thin wings, with a friend, and mentioned that
I needed some "C" grade balsa to make the wings.  He had never heard
of balsa grades, and thought that "balsa is balsa".  Here's a web site
I found that gives a simple descroption if the grades and their
properties; picture and text are at the bottom of the page.

http://www.rcscalebuilder.com/tutorials/Balsa/balsa_1.htm