A habit is something that you may or may not realize. For example, you have a habit of drinking water in the morning. This is something that you consciously do, and this is part of your routine. You may also bite your nails. This is a habit that you may not consciously know.
An addiction is something that you do that you cannot live without anymore. For example, you are addicted to gambling. You are willing to sacrifice all of your belongings, all of the things that you have invested just to fuel your addiction. Being addicted to something, especially a vice, will always have negative results.