Film Noir (literally, 'black film'), despite being harder to define than it might appear at first, is one of the most profoundly important genres / styles / aesthetics (see? It's difficult to say what it even IS!) in cinema. In fact, a fairly typical list of noir movies, like
this one, would make a pretty good introduction to the history of film. Although its influence is discernible all over the world (particularly in these globalised times), it is probably best known in its American incarnation (especially in the 'golden age' of the 1940s and 1950s) and that's where we'll be focusing.
Aesthetically, it depends heavily on chiaroscuro lighting, unusual camera angles and wide-angle lenses. Much of this is derived from
German Expressionism. Plots, originally, were often taken from the hardboiled detective stories of writers like James M. Cain, Dashiell Hammet and Raymond Chandler (all of these writers are well worth investigating, especially (in my opinion) Cain. Start with
The Postman Always Rings Twice,
Mildred Pierce and
Double Indemnity , all later filmed to outstanding effect. If you don't read the books, AT LEAST watch the movies.) Stock characters - the hard-drinking gumshoe detective, the treacherous femme fatale, the cops and criminals who are often hard to tell apart - are borrowed from those same books and given life by many of the most famous actors of the era. The location, as is typical of the Crime matrix genre, is urban and tends to focus on seedy office, alleys, clubs and so on. Narratives often depend upon first-person voice-over, flashbacks and (of course) more-or-less Todorovian, 'classic Hollywood' three-act structures. As such, it seems to have all the things demanded to qualify as a 'proper' genre - Buscombe's iconographic categories (location, tools, appearance and miscellaneous(!)) are all well satisfied - and there are recogniseable conventions which allow for the establishment and satisfaction of foreknowledge and expectation in the audience.
Less specifically, critics Raymond Borde and Etienne Chaumeton (1955,
A Panorama of American Film Noir) offered a definition of noir as '...oneiric, strange, erotic, ambivalent, and cruel...' ('Oneiric' means 'dreamlike'.) They admit themselves that this is unsatisfactorily simplistic and reductive, but it is a useful and interesting starting point. A 'typical' noir will have a sexual charge (usually between the detective and the femme fatale), a lot of unthinking and often apparently motiveless cruelty, and, often, nothing resembling a 'happy' ending. They aren't cheery films.
So why the argument about whether noir is actually a genre or not? Well, though we've identified a lot of conventions, a huge number of these films irritatingly refuse to follow them. For example, probably the most recognisably 'noir' characters are the detective and the femme fatale, but most noir movies feature neither. Equally, though the genre (or whatever it is) is strongly associated with an urban setting, many of these films have small-town or semi-rural settings. What is perhaps more consistent, however, is the aesthetic of the movies. Monochrome, low-key lighting, creative use of shadows, claustrophobic, low-ceilinged sets, Dutch or canted angle shots, lots of crane shots and low-angle shots, highly technical pans and tracks - all are strongly associated with noir, and are perhaps what makes it a recogniseable style and, indeed, so influential today. It may be that we love these movies not so much for their character, plots or narratives, but for their appearance.
There are far too many 'great' noirs to offer a comprehensive list, but classic examples are Billy Wilder's
Double Indemnity (1944), Tay Garnett's
The Postman Always Rings Twice (1946) and Robert Aldrich's
Kiss Me, Deadly (1955). A (fairly random) scene from each should give some feel for noir. (By the way, Hitchcock often gets mentioned as a noir director. He did make movies which are fairly clear examples, but his movies tend to be (again, in my opinion) less easily defined - they contain too many elements of horror, too much humour and too many narrative 'tricks' to allow them to be categorised as such.)