A peer-reviewed (or refereed) article has been read, evaluated, and approved for publication by scholars with expertise and knowledge related to the article’s subjects and contents. Peer-reviewing helps insure that articles provide accurate, verifiable, and valuable contributions to a field of study.
A technical defintion...
"That which is produced on all levels of govenment, academics, business and industry in print and electronic formats, but which is not controlled by commercial publishers."
(Fourth International conference on Grey Literature, Washington D.C., October 1999)
Examples of grey literature may include:
Market research reports
Newsletters and/or bulletins
Dissertations & Theses
Examples of why you might use grey literature: