What are the differences between a habit and an addiction?

Answer 1

A habit is an action or behavior that is repetitious in one's life.

An addiction -- is a craving, a substance, person, behavior, ritual that a person feels must be done, that is essential to one's quality of life and supersedes all other things.

Answer 2

An addiction generally refers to a chemically induced dependancy like nicotine, caffine, alcohol, or types of pharmacuticals. Where as a habit is usually refering to an action that has become unconciously routine, often stress related or induced, like nail biting, hair twirling, lip biting,hand wringing etc.
Hope that helped!