C-Files are naturalization certificate files. These files came into existence with the Basic Naturalization Act of 1906 and the creation of the Federal Naturalization Service (FNS). The job of the FNS was to collect and maintain duplicate copies of all court naturalization records. Everyone who was naturalized between 1906 and 1956 has a C-File, which includes a copy of the declaration, petition and certificate of naturalization.
How to Request Copies of C-Files for Genealogy Research | eHow.com