I want to know what is the difference between a stoner and a drug addict or junkie?

Essentially none: All these terms have one common theme, the use/abuse of a perticular drug. These derogatory terms reference those abusing drugs.

Addiction =addiction: These are all terms for people affected by addiction. Some of them sound "cuter" than others, but they all mean the same thing. I once know someone who preferred to call him self a pothead rather than a marijuana addict, but he was just that, a marijuana addict.