A Knight is a man or woman of honor, one who traditionally follows the Knight's Code. However, Dark Knights usually don't follow it, and if they do, they follow a reversed version where they accept rewards and disrespect women, as well as have sex as they please.
An example of a Dark Knight brotherhood is The Return .
A knight is also a character archetype, where a character must be chivalrous. The Dark Knight archetype is the opposite of this, where you do whatever the fuck you want.