There are several situations when certain pattern of behavior can be formed in a person. That is called addiction. A modern medicine now makes a difference between psychological addiction (which is called simply addiction) and physical dependence. Physical dependence is caused by hard drugs or alcohol for example. There are many other substances that can cause physical dependence. Addiction is emotional dependency of mind and all the symptoms are psychological. So such addiction can be formed by any behavior that can be pleasant for the person (it is usually called rewarding behavior).
No matter what type of addiction a person has, physical or psychological, it is always a huge problem as it can seriously harm and even ruin the entire life. So, if any signs of addiction are present special treatment should be applied.
Today doctors use different medical models of addiction. These medical models assume that addiction is a result of certain physical condition. In that sense most of the medical models of addiction refer to physical dependence like alcohol addiction. Usually these kinds of dependencies and medical models involve so-called withdrawal symptoms - those that come when affection of stimulus that caused the dependencies is stopped. Of all the medical models of addiction the most striking example is tobacco addiction.
Another pretty close to most of the medical models is disease model of addiction (some doctors even consider it as a kind of medical models). In that model addiction is described as a life-long disease in which patient loses control over his/her behavior (and so both medical and spiritual help is needed for that patient). As this model describes addiction can be inherited from parents or acquired through the life experience.
But this model does not show any biological mechanism to determine such behavior and its roots. And in most cases this is the main argument for critics of disease model of addiction. And people who speak against this model of dependency also claim that such an approach has a very negative social effect and thus does not help to treat addiction at all.
There is also so-called genetic model of addiction that claims that certain pattern of behavior is determined by genes. That means that all the dependencies are inherited and thus they need special medical methods of treatment.
Different sorts of social model of addictions are also very popular. In all such models it is assumed that addicts' behavior patterns are formed by society. These models are especially popular these days to explain a new kind of dependency - Internet Addiction. There are even special centers that are oriented on treatment of online addictions - in most cases they use such models in their treatment techniques and methods.
Term addiction is also used sometimes for people who have a passion for something - for example, term workaholic. But in fact these cases have nothing to do with real addictions and dependencies and a special model cannot be applied here.