Film Noir is a genre of stylish crime dramas, difficult to define, but the 1940's and 50's were the classic period. Whether works since then can be accurately classed as Noir is a subject of much debate among film critics. Film Noir, and the literature from which it is drawn, is clearly the progenitor of later genres, particularly Cyberpunk.
Common subjects of noir films include murder investigations, heists, con games, and (mostly) innocent men or women Wrongly Accused of crime. The double-cross and cigarette smoking are mandatory. Complicated plots are further convoluted by Flashbacks and Flash Forwards—the narration tying everything together, assuming we can trust him.