A need is something you have to have, something you can't do without. A good example is food. If you don't eat, you won't survive for long. Many people have gone days without eating, but they eventually ate a lot of food. You might not need a whole lot of food, but you do need to eat.
A want is something you would like to have. It is not absolutely necessary, but it would be a good thing to have. A good example is music. Now, some people might argue that music is a need because they think they can't do without it. But you don't need music to survive. You do need to eat.