Grey literature (read a helpful definition here) is not published in traditional commercial venues nor included in major databases. It can take the form of research reports, theses and dissertations, newsletters, bulletins, pamphlets, blog entries, and more.
Grey Literature Report - From the New York Academy of Medicine
Research Guide to Grey Literature - History of Medicine - NYU Libraries
Criteria to keep in mind when choosing and using soures:
Accuracy - Does the author cite her/his sources and are they legitimate?
Authority - Who wrote the source? Are they credible?
Objectivity - Does the author have a bias, political or commercial or persuasive?
Currency - Is this information new or based on outdated sources? Can you tell how current it is?
Office of Minority Health (US Dept of H&HS)
Boston Women's Health Book Collective aka Our Bodies Ourselves