Grey Literature is literature (often of a scientific or technical nature) that is not available through the usual bibliographic sources such as databases or indexes. It can be both in print and, increasingly, electronic formats.
Grey literature is produced by government agencies, universities, corporations, research centers, associations and societies, and professional organizations.
Many common reference examples are provided in Chapter 7 of the APA manual
Grey literature is an important source of information. Though not scholarly, it is produced by researchers and practitioners in the field. It can often be produced more quickly, have greater flexibility, and be more detailed than other types of literature. "Grey literature serves scholars and lay readers alike with research summaries, facts, statistics, and other data that offer a more comprehensive view of the topic of interest (Weintraub)."
In the future, grey literature will be even more important. "In a world in which free trade and instantaneous communication have eliminated many of the barriers to information flow, grey literature is gaining greater importance as a source of information for much of the world's population (Weintraub)."
For more information about grey literature, see IL Toolkit - Finding Information: Gray Literature or The Role of Grey Literature in the Sciences by Irwin Weintraub.