The phrase is French for "deadly woman", usually 'A seductive woman who lures men by an aura of charm and mystery into dangerous or compromising situations to achieve her hidden purpose '. In some situations, she uses lying or coercion rather than charm. Her ability to entrance and hypnotize her victim with a spell was in the earliest stories seen as being literally supernatural; hence, the femme fatale today is still often described as having a power akin to an enchantress, vampire, witch, or demon.
Although typically villainous, femmes fatales have also appeared as anti heroines in some stories, and some even repent and become heroines by the end of the tale. In social life, the femme fatale tortures her lover in an asymmetrical relationship, denying confirmation of her affection. She usually drives him to the point of obsession and exhaustion so that he is incapable of making rational decisions. Her charms leach the virility and independence of lovers, leaving them shells of themselves.
To encapsulate the persona of a Femme Fatale, I did a little research into the actresses which played female characters that represented these traits, often found in 40's-50's Film Noir, putting together an Influence Map to refer to when creating Yuki-Onna