The Twelfth International Conference on Grey Literature in Prague in 2010 arrived at 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."
Traditionally, the term gray literature has included:
There are also many other forms of gray literature, including: Newsletters, technical notes, working papers, white papers, patents, and more.
The name doesn't make it especially appealing, perhaps bringing to mind the image of dusty stacks of outdated reports with titles so arcane that one 
might be prompted to wonder if they were ever read, even when new. So why go delving into what can be daunting territory?
A thesis may contain data that is never included in the journal article that is ultimately published using its findings. In other cases, a broader view may be what is wanted, in which case a government factsheet or institutional newsletter targeted to a lay audience may meet the searcher's need. It is also important to note that, due to various forms of publication bias, including positive results bias and time lag bias, studies with negative results are far more likely to be found in gray literature than in mainstream publications.
Results of studies may appear in gray literature 12 to 18 months before being published via traditional channels.
Rather than waiting years for the publication of a revised edition, authors, editors, and Web content creators can update information when needed, a factor that reinforces the timeliness of gray literature.
Although governments and industries often restrict the readership of certain types of gray literature (classified or proprietary information, for example), there is also a great abundance of gray literature that is freely available to all, either in print or on the Web.
It takes nothing away from gray literature to note that its use may involve some difficulties, including:
While there is a great deal of gray literature that is freely available, it is often produced for a narrowly targeted 
readership, without the goal of gaining a wide audience. This can make it hard to index and catalog, which increases the difficulty of finding it. Furthermore, one person's flexibility is another's instability; something that was here (on the Web) today may be gone tomorrow.
You may need to free yourself of the mindset promoted in some academic disciplines of looking only to peer-reviewed work published in "reputable" journals. There is a great deal of information produced by both commercial enterprises and government agencies which lies outside of the scholarly realm yet is of high value.
You may be quite familiar with the citation styles for books and journal articles but feel mystified when faced with the need to cite a white paper, newsletter, or dissertation. Most if not all of these are discussed and examples given in the APA and other style manuals. Also, feel free to ask a librarian for help with this. Don't let a citation question stand in the way of referencing a valuable resource!
Although this may not initially appear to be an end-user issue, it will ultimately become one. Libraries and other information repositories are faced with the huge challenge of choosing, out of the vast expanse of gray literature, what should be collected and in what medium/format it should be preserved. Likewise, in the print world, storage space can quickly become a limiting factor in the retention of gray literature. Remember: GRAY does not always STAY.
Start by asking yourself "who cares" about your topic. What agencies, organizations, or associations cover this subject? Consider advocacy groups, government and private agencies, industries, and trade/professional associations.
If you're unsure and need to begin searching in a more general way, here are some places to begin:
From www.greyorgray.com:
grAy is how it's spelled in America
grEy is how it's spelled in England
