"Information produced on all levels of government, academia, business and industry in electronic and print formats not controlled by commercial publishing i.e., where publishing is not the primary activity of the producing body" -ICGL Luxembourg definition, 1997- Expanded in New York, 2004
"Gray literature...refers to informally published written material (such as reports) that may be difficult to trace via conventional channels such as published journals and monographs because it is not published commercially or is not widely accessible." - Wikipedia definition of "grey/gray literature"
Types of grey literature include web 2.0 communication channels, ePrints, preprints, post prints, email, texts, listserv archives, wiki articles, images, theses, dissertations, conference proceedings and abstracts, clinical trials and practice guidelines, technical reports, standards, patents, newsletters, videos, etc.
Finding grey literature can be difficult and require exhaustive searching. Since grey literature is not included in bibliographic databases, it requires different search techniques in order to find information relevent to your topic of interest. The links below offer quick introductions and tutorials, but a librarian is always a great place to start!