Generally-speaking, not all abuse is addiction, but all addiction is abuse.
When we abuse something we are improperly using it. Addiction is a misuse of something, both a misuse of the subject of the addiction and a misuse of the addict's life. For example, alcohol is not made to be drunk in excess, and neither is the human body designed to consume mass quantities of alcohol.
Habits can also look like this same mutually-distorted behavior. Many people are racked with bad habits (check out our prisons if you have any doubts). Drug and alcohol abuse often begin as 'bad habits,' but then go beyond. Here are some ways to discern:
1) Habits are often 'thoughtless,' done without a great deal of thinking. Addictions usually engage a great deal of thought to excuse the behavior or make the addiction possible.
2) Both habits and addiction inhibit impulse control (or are caused by it, which is why young people are the likely candidates for both), but habits can be 'unlearned.' Addictions cannot.
3) Habits are usually conditional, only happening in certain circumstances. Addicts, however, obsess about the addiction regardless of circumstances.
4) If a person begins to break a habit, he will feel discomfort if in the 'trigger condition' in which he usually engages in the habit, but will overcome the anxiety if he does not give it. The addict will not only feel the discomfort, but will not feel alleviation until he engages in the addiction or a replacement addiction.
5) A person can often catch himself in a habitual act and break it, whereas an addict cannot stop his addiction once he starts.
As with all things, this is an over-simplification, and the human condition is often complex. But, one should not be too quick to call someone else an addict. Lots of people have bad habits these days, and so we must be certain that the abusive behavior really is an addiction... and not just a bad habit the other person is willingly engaging in.