Adapting a definition from GreyNet International, Gray Literature (also spelled "Grey Literature") refers "...to multiple document types produced on all levels of government, academics, business, and organization in electronic and print formats not controlled by commercial publishing i.e. where publishing is not the primary activity of the producing body."
Because these document types are produced and published by a myriad of non-commercial entities, many of them do not end up being indexed bibliographically. This means that you cannot find them in the usual subject-specific databases used for scholarly purposes.
Technical reports, often produced by government agencies, are a commonly utilized form of Gray Literature in engineering and the sciences.
As discussed in this guide from the Science and Research Services at the Library of Congress,
"These reports are issued for a variety of purposes: to communicate results or describe progress of a research project; as background information on an emerging or critical research topic; to provide a list of instructions or procedures for current practices; to determine the feasibility of a technology and recommend if the research should be continued (and how to evaluate progress that is made); and finally, to detail technical specifications (materials, functions, features, operation, market potential, etc.). "
By their nature, technical reports often include a level of detail of interest to a very specific, technically-aware audience. Therefore, technical reports have a very different function than research journal articles that mostly discuss methods and procedures cursorily and focus on findings. These details can be exceptionally valuable to the professional engineer or technologist, for example.
The Technical Report Archive & Image Library (TRAIL) identifies, acquires, catalogs, digitizes and provides unrestricted access to U.S. government agency technical reports. "The mission of TRAIL is to ensure preservation, discoverability, and persistent open access to government technical publications regardless of form or format."