They are patterns of sexual behavior whose predominant source of pleasure is not found in copulation, but in some other activity or thing that accompanies it. They do not have to be negative behaviors, but since they depend on social conventions prevailing at a certain time and place, they must be placed in their context. In fact, masturbation and oral sex were considered paraphilias until the mid-20th century.
They usually, although not necessarily, happen mainly because the person who practices them has already had a very high amount of sexual pleasure, that there comes a time when the little does not satisfy them and they want more and more of that activity to feel the orgasm or get excited.
The most common definitions include behaviors such as sadismhe masochismhe exhibitionismhe voyeurismthe zoophiliathe coprophiliathe necrophiliahe fetishism and the frotism.