Grey Literature includes sources that do not fall into "traditional" publishing categories: theses/dissertations, government documents/reports, white papers, patents, conference proceedings, newsletters, unpublished or unfinished clinical trials, etc.
The 12th International Conference on Grey Literature (Prague, 2010) provided the following definition:
"Grey literature stands for manifold document types produced on all levels of government, academics, business and industry in print and electronic formats that are protected by intellectual property rights, of sufficient quality to be collected and preserved by libraries and institutional repositories, but not controlled by commercial publishers; i.e. where publishing is not the primary activity of the producing body."
Using "site searching" can be a useful way of finding grey literature (especially government reports).
Examples of site searching:
Other Google resources that could be useful are linked below: