"Film Noir" is a term that was applied to a style of American cinema that was popular in the 1930's and 40's. The term translates to "Black Film," which refers to both the characteristic lighting and the dark subject matter. Noir films often depict different aspects of the criminal underworld, and are most commonly set in the 'mean streets' of the city.
Many of the most recognizable early noir films were mysteries involving a hard-boiled detective like Sam Spade (played by Humphrey Bogart) who gets involved with a woman who hires him to delve into the criminal underworld to solve a case. Many of these tales are based on dime-store novels (also known as "pulp fiction") written by authors like Dashell Hammett ("The Maltese Falcon").
Other popular noirs of the period often tell the tale of an average joe who is put into a difficult situation that continues to get worse, pushing him to his ethical, mental, physical, or moral limits.
Noir is most distinguished by its effective interplay of light and shadow. Noirs are lit very darkly, with beams of light being used to highlight certain important elements of the scene, actors, or action. For example, a hero character would be well-lit, while the villain lurked in the shadows. Someone who was of a questionable nature could pass in between the light and shadow, perhaps with a diagonal shadow hiding their eyes while the rest of their body is well-lit.
One of the biggest requirements for a noir was the presence of the femme-fatale character. This was often a very beautiful woman, attractively dressed, who seduced the hero into her world of darkness like a black widow spider. She could be the detective's client, the gypsy who knows a little too much, the girlfriend or wife of the protagonist, or commonly the villain in the story.
Noir films continue to be made today, but purists often dub them "neo-noir," saving the term "film noir" for the period of films between the 1930's and 1950's. Some popular examples