Similar to, and most recognizable as an abbreviated form of, Who does she think she is?. This abbreviated version, however, is a bit more existential in its search for an answer because it is more open-ended. The question does not ask what said person thinks of herself, but instead asks the greater cosmos (or anyone included in or surrounding the conversation) why that girl exists and what she purportedly does to maintain any usefulness in society. It is generally degrading to use this term, so don't use it in front of the person you are referencing. Use intonation (raising your voice at the end of the sentence) to denote not only the question part, but also your insufferable hate of this person. Make sure to maintain stress, and some whining, on the IS. Can be used for males as well, e.g. who is he? But other forms such as "who are we?" and "who am i?" are more comical in reference and do not hold the same connotation as "who IS she?"
person 1: omg, did you see her trying to sweet talk the boss this morning? and she totally sucks at her job, i think a kid almost drowned yesterday and no one said anything to her!"
person 2: who IS she?
person 1: i know! If i had done that, i would be fired already!