Primary Sources - the records and artifacts of an event that were produced and gathered by people who witnessed or heard of the event at the time.
In film studies, Citizen Kane could be a primary source for many projects -- Orson Welles' career, Hollywood in the 1940s, movies about newspaper men, or the RKO studio. The original scripts, financial records, posters, stills, props, and productions notes for the movie are also primary sources.
Museum archives, library special collections, and other places collect primary sources, preserve them, and make them available for popular research. Most often, researchers must go to the library or archive to view primary sources in person. A few examples of archives and specail collections:
Academy of Motion Picture Arts and Sciences Film Archive, a museum archive of winners and nominees in Los Angeles. http://www.oscars.org/filmarchive/index.html
National Board of Review of Motion Pictures Papers - a specical collection at New York Public Library. http://www.nypl.org/research/chss/spe/rbk/faids/nbrmp.pdf
UCLA Film and Television Archive - at the University of California - Los Angeles, a large archive that includes film and TV. http://www.cinema.ucla.edu/
King Vidor Papers a special collection of the personal and business papers of the director at the University of Texas-Austin. http://www.hrc.utexas.edu/research/fa/vidor.html
To search for the existence of primary sources in the U.S. for film research :
National Union Catalog of Manuscript Sources a archive and collections index sponsored by the Library of Congress. http://www.loc.gov/coll/nucmc/rlinsearch.html
To search for the existence of primary sources for international film research,
Use the FIAF databases.
Secondary Sources works of scholarly interpretation and analysis based on the exploration and use of primary sources. To search for secondary sources in film studies, look in academic journal articles, books, and all the other places listed above.
It is in these sources that you will find the film reviews, biographies of directors, articles of criticism, and histories that bear on your work.