Grey literature is information that falls outside of the normal publishing realm.
Examples of grey literature include technical reports, patents, dissertations, theses, pre-prints, some conference proceedings, as well as working papers.
Because grey literature is produced directly by researchers in the field, it is often scientific or technical, since it is written for a very specific audience. And since grey literature doesn't go through the normally lengthy publication cycle, it may be more up-to-date than information published in more traditional formats.
Sources listed to the right will provide access to grey literature.
Image source: Grey fungus. SAB ATH. CC BY-SA 2.0. Wikimedia Commons.