Film noir is a cinematic term used primarily to describe stylish Hollywood crime dramas, particularly those that emphasize cynical attitudes and sexual motivations. The term film noir, it's French for 'black film'.
Classic film noir developed during and after World War II, taking advantage of the post-war ambience of anxiety, pessimism, and suspicion. Hollywood's classic film noir period is generally regarded as stretching from the early 1940s to the late 1950s. Film noir of this era is full of urban crime and vice.
Film noir has a distinct style, with shadow-filled low-key lighting. Night rules in film noir, where the shadow more important than the light in the general style of the mood, although light is crucial as it highlight the most important aspect in the single frame, for example, gun.
The primary moods of classic film noir were melancholy, alienation, bleakness, disillusionment, disenchantment, pessimism, ambiguity, moral corruption, evil, guilt, desperation and paranoia.
Strictly speaking, film noir is not a genre, but rather the mood, style, point-of-view, or tone of a film. It is also helpful to realize that 'film noir' usually refers to a distinct historical period of film history - the decade of film-making after World War II, similar to the German Expressionism or the French New Wave periods.
The heroes, corrupt characters and villains included down-and-out, conflicted hard-boiled detectives or private eyes, cops, gangsters, government agents, a lone wolf, socio-paths or killers, war veterans, politicians, petty criminals, murderers, or just plain Joes. These protagonists were often morally-ambiguous low-lifes from the dark and gloomy underworld of violent crime and corruption. Distinctively, they were cynical, tarnished, obsessive (sexual), brooding, menacing, sinister, sardonic, disillusioned, frightened and insecure loners (usually men), struggling to survive - and in the end, ultimately losing.
On the other hand, femma fatal in film noir were either of two types - dutiful, reliable, trustworthy and loving women, or femmes fatales - mysterious, duplicitous, double-crossing, gorgeous, unloving, predatory, tough-sweet, unreliable, irresponsible, manipulative and desperate women. Usually, the male protagonist in film noir wished to elude his mysterious past, and had to choose what path to take.Invariably, the choice would be an overly ambitious one, to follow the dangerous but desirable wishes of these dames. It would be to pursue the goadings of a traitorous, self-destructive femme fatale who would lead the struggling, disillusioned, and doomed hero into committing murder or some other crime of passion coupled with twisted love. When the major character was a detective or private eye, he would become embroiled and trapped in an increasingly-complex, convoluted case that would lead to fatalistic, suffocating evidences of corruption, irresistible love and death. The femme fatale, who had also transgressed societal norms with her independent and smart, menacing actions, would bring both of them to a downfall.
Storylines were often elliptical, non-linear and twisting. Narratives were frequently complex, maze-like and convoluted, and typically told with foreboding background music, flashbacks (or a series of flashbacks), witty, and/or reflective and confessional, first-person voice-over narration.
Overall, I can conclude the film noir films to be (mostly shot in gloomy grays, blacks and whites) thematically showed the dark and inhumane side of human nature with cynicism and doomed love, and they emphasized the brutal, unhealthy, seamy, shadowy, dark and sadistic sides of the human experience.