Lumber grades explained

[Tom Walz]

http://www.artofmanliness.com/2015/03/04/primer-on-lumber/

[dan sherman]

This is a simplified & illustrated guide.
http://www.ahec.org/hardwoods/pdfs/IllustratedGradingGuide.pdf

And if you want to know way to much about grading you can read the grade book.
http://www.nhla.com/assets/1603/2011_rules_book.pdf

[Randy Red Bemont]

A lot of good info here.

Red