Psychologists group abnormal behavior into two main categories: atypical behavior that is not necessarily harmful to oneself or others and maladaptive behavior that is potentially harmful. Some types of behaviors are both atypical and maladaptive.
Atypical behavior may be interpreted by others as weird, strange or deviant, but it may not necessarily be harmful. It can include behavior such as talking to the paint on the wall or laughing all the time. The word "deviant" in this context means it deviates from normal behavior.
Maladaptive behavior can be harmful to people, not just physically but also mentally or emotionally. An example of atypical and maladaptive behavior is dancing naked on a railing of a highway overpass. The behavior may be perceived as weird, and it is maladaptive because there is potential for harm, if the individual falls. Some types of maladaptive behavior are not deviant, like drinking oneself into a stupor for instance.
Behavior that is both atypical and maladaptive can be criminal, such as child molestation. It is atypical because it deviates from normal sexuality, and it is maladaptive because it harms the child emotionally and possibly physically.
Other psychologists include the categories of norm violation, statistical rarity and personal discomfort under abnormal behavior. A norm violation is doing something contrary to norms, which are the social rules that guide behavior. A statistical rarity is something that deviates from the average. Behavior may be perceived as abnormal, if a person feels personal discomfort over his actions or thoughts.