Actually, I am tempted to answer: "Film noir is a label which film critics use merely because it sounds good." The correct pronunciation is [film nwar] by the way. Originally, it is a French expression, meaning "black cinema". The term was borrowed from "roman noir", gothic horror stories from 19th century England.
The thing is that film noir cannot easily be defined. There are a few movies which most cineastes and critics label film noir, though. I would like to divide these movies into four different categories:
French film noir (1930-1940): The term refers to dark movies from especially the late '30s, e.g. Quai des brumes (1938) and Le jour se lève (1939) by Marcel Carné, which gained international reputation at the time. French film noir is characterised by poetic realism and cruel fatalism, a down-to-earth doomsday feeling, so to speak. In this context, the term film noir is used in a more or less frequent manner in many European countries, but not necessarily in France.
Early American film noir (1930-1940): Many cineastes and critics label early crime stories from mainly Warner Brothers as film noir, like Little Ceasar (1930) and I Am a Fugitive from a Chain Gang (1932). Although we would regard them as very conventional and very harmless today, they shocked the audience in the '30s and actually got banned in some countries. It is questionable if those movies really are true film noir.
American film noir (1940-1950): American film noir commonly refers to classic gangster movies from the '40s and '50s, often adaptations of hard-boiled, contemporary pulp fiction, e.g. by Raymond Chandler. Classic examples of American film noir are The Maltese Falcon (1941), Kiss of Death (1947), The Naked City (1948), The Asphalt Jungle (1950), Kiss Me Deadly (1955). A complete site could easily be devoted to this category alone.
Modern film noir (1950-): This is not really an established label, but I think it is suitable. To this category I would