What is the difference between need, want and demand?

Need is your necessity without which you cant live, for example, Food, shelter, clothes. 

Wants are the requirements which can add up a certain comfort level in your life, like furniture, transport, etc. 

And demand are your wishes, which you always run before so they can come true, e.g. buying a sports car, owning a Jet, having a nice girl friend perhaps :D