Technical Reports are part of Gray Literature or "Grey Literature."
Their main characteristics are:
TRAIL Technical Report Archive & Image Library (TRAIL) is an initiative led by the University of Arizona in collaboration with CRL and other interested agencies to identify, digitize, archive, and provide access to federal technical reports issued prior to 1975.
Virtual Technical Reports Collections (University of Maryland Libraries provides links to full-text or searchable extended abstracts of technical reports, preprints, reprints, dissertations, theses, and research reports in all disciplines.