What is luxury and necessity?

As per my knowledge goes, the need and necessity is a similar condition where the difference between the two lies on the priority. A need is an absolute craving of man to possess while the necessity is also a craving without which man manages to survive. For example transport is a need while the personal vehicle is a necessity. Here transport is a need of mankind to move from one place to another without which there will be hindrance in his daily affairs which may effect his existence also. But a personal vehicle is only a luxury which will enhance his ability to move. He may move without the personal vehicle by using alternate means. The term desire is applicable to the craving for the fulfilment of both the set. But I feel the desire that is having a negative effect on one's life is the craving for the set of necessity.