Film Noir is not exactly a genre, not only a historical movement in film history. It is something we know when we see. Paul Schrader has written one of the best overviews of what a Film Noir actually is in his Notes On Film Noir (1972).
Now the BFI has produced an entertaining and enlightening infographic about the elements that make a film noir. It leads the reader through the various elements and attitudes of noir and it even has a pinboard that attempts to determine which is the most Noir Film of all time.
It's fun. Check it out here.