Gray literature refers to any information, aimed at academic or professional audiences, that isn't directly from a traditional publisher. This is a very broad definition that basically includes anything other than books, journal articles, magazine articles, and newspaper articles. Here are some examples of good sources that would be considered gray literature:
If you'd prefer a more official definition, the Twelfth International Conference on Gray Literature defined it as:
"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."
Unfortunately, there is a lot of misinformation disguised as gray literature out there, and some very useful information that looks unprofessional at first glance. Try using these helpful techniques to distinguish the good information from the bad.