The term grey literature is used to describe a wide range of different types of information that is produced outside of traditional publishing and distribution channels, and which is often not well represented in bibliographic databases. The Fourth International Conference on Grey Literature (GL '99) in Washington, DC, in October 1999 defined grey literature as follows:
"That which is produced on all levels of government, academics, business and industry in print and electronic formats, but which is not controlled by commercial publishers."
Grey literature can take the form of reports, conference proceedings, clinical trials, official publications and theses. In general, grey literature is not peer-reviewed. The absence of a peer-review process speeds up publication, so grey literature can contain very timely information. Grey literature sources can vary hugely in terms of quality, and you should exercise caution and make your own assessment for quality, reliability and potential bias.
Conference abstracts can be found in Google Scholar, and in many databases e.g. Scopus, Web of Science, CINAHL and IEEE Xplore.