Disease. Drug or alcohol use may start as choices, but addiction is a set of neurobiological (brain) changes that you don't get to vote on. Just like you can choose to eat sugar, but if you develop uncontrolled blood sugar levels (diabetes), you can impact but not reverse the disorder. See http://www.Asam.Org/research-treatment/definition-of-addiction for more information.
Depends who you ask. If you are a victim, then its a crime. If you are a treating psychologist, it is a psychiatric condition. If you are a lawyer, it is a legal definition. You can't blame bad choices on addiction. Addiction leads to bad choices.