First of all,what is noir? I was confused when first time I hear the word 'noir'.
Then,I did so many researches about noir,so,here is my understanding about noir.
According to Dictionary,Noir showing characteristics of a film noir,in plot or style.It also means that having the characteristics of film noir; tough and bleakly pessimistic. http://www.thefreedictionary.com/noir

Baiscally,noir contains of top 5 elements.And also please remember noir can be found in any media, be it art, literature, film, video-games, or comics, it's there.

1. The Seedy Underworld

The setting of any great noir piece is the filthy underbelly of the polished public. This place is filthy, grimy, bleak, and often deranged. Expect steaming sewer grates, torrential downpours, second-hand-smoke, and nefarious characters. Crime thrives here, and its perveyors are brutally corrupt and extremely adept. Sadly, these roads are lined with condemned buildings labeled 'hope,' 'faith,' and 'charity.'

2. The Anti-Hero

The main character of the noir genre is at best despicable. We are fascinated with his/her moral ambiguity and glaring flaws while hiding our disgust with the same. Dangling from this character's lips is the tell-tale cigarette and the greasy tendril of smoke tickles the street lamp above. Usually this character's life is at stake, and this is almost always due to their own poor choices. We quietly root for him/her and cringe when the baseball bat or crowbar beats their brow bloody.

3. Femme Fatale

A secret ingredient that adds spice and variety to any noir is the femme fatale. She is sultry, sexy, and manipulative. Her power over the main character is unmistakable and she gently leads him along to meet her own ends. Elegance of speech and brutal double crossings as common as lipstick stains on wine glasses and discarded lingerie. She is deadly, untrustworthy, and 9 times out of 10 the villain.

4. Misogyny

Throughout the genre of noir, there seems to be a fairly steady stream of misogyny. I'm not suggesting that this "dame-hating" is always aware of itself or that it's a conscious decision by the noir creators, but it rears itself very consistently in each piece. Women are the villains, the burdens, and the childish fools that merely tolerated by the hero. Also the women are always "male-defined." Their behaviors, attitudes, and appearance are constantly pandering to a male audience. I assume that this must be incredibly frustrating for female audiences (it frustrates me and I'm a male). As a result of these male definitions, noir seems to always focus on beautiful women with low sexual standards and no depth of character (other than where it serves plot). Recently I've read that there are some female authors who have reversed the roles in modern noir novels. I hope to read some of them soon so I can report back my findings.

5. Redemption

Another theme that has presented itself in my studies is that of redemption. In noir, the detective always arrives after the tragedy has taken place. He is plunged into a world of chaos, where something whole was just shattered (murder, rape, violence, etc.). His job is completely consumed with trying to redeem the unredeemable. Think Batman. Bruce Wayne is haunted by the murder of his parents. He decides to seek vengeance for the rest of his life by "righting" all the wrongs in Gotham. The only catch is that his task is impossible. He cannot change the fact that his parents are dead, nor can he completely succeed in purifying Gotham. He can' t win. The only measure of success he achieves is through putting as many pieces back together as he can. In this way we (the audience) are lead to believe that something has actually been redeemed. We feel better about the injustice when we know how or why it happened, even though it still occurred. Dean DeFino calls this "illusions of order" and "intellectual control." He says that "the story redeems that sense of order and control by (fictionally) exposing its logic, its cause-and-effect chain, how one thing leads to another."

References from http://noirwhale.com/category/noir-definition/

Furthermore, the light,shadow,and dramatic cinematography play a vital role in a completed noir work.
Film Noir