A paraphilia is a condition involving sex fetishes where a person's sexual arousal and gratification depend on fantasizing about, and engaging in, sexual behavior that is atypical and extreme. A paraphilia can revolve around a particular sex fetish like an object (e.g., children, animals, underwear) or around a particular sex fetish act (e.g., inflicting pain, exposing oneself).

Most of the paraphilias are far more common in men than in women. The focus of a paraphilia is usually very specific and unchanging. For example, for someone who derives sexual pleasure from exposing his genitals, watching others engaging in sexual activity will not generally provide sexual gratification.

A paraphilia is distinguished by a preoccupation with the object or behavior to the point of being dependent on that object or behavior for sexual gratification. In most cases, types of sexual activity outside the boundaries of the paraphilia lose their arousal or satisfaction potential unless the person fantasizes about the paraphilia at the same time.

Although many of the paraphilias seem so foreign or extreme that one could not imagine how the object or behavior can be arousing to anyone, they are easier to understand if one thinks of those behaviors that, in less extreme versions, are quite common and not considered abnormal. For instance, having a partner "talk dirty" occasionally may be a "turn-on" for some people, but when talking dirty is the only way that sexual arousal or satisfaction can occur, it would be considered a paraphilia.

Others want to be bitten, scratched, or spanked, or find that watching their partner undress is highly arousing. Viewing a nude person or watching sexually explicit videos can be arousing for most people. Each of these acts is innocuous unless magnified to the point of psychological dependence.