Paraphilia is the clinical term used to describe and diagnose a particular kind of sexual disorder. The Diagnostic and Statistical Manual of Mental Disorders (DSM) defines paraphilias as:
recurrent, intense sexually arousing fantasies, sexual urges, or behaviors generally involving 1) nonhuman objects, 2) the suffering or humiliation of oneself or one's partner, or 3) children or other nonconsenting persons that occur over a period of at least 6 months.
The DSM only specifies nine paraphilias (exhibitionism, fetishism, frotteurism, pedophilia, sexual masochism, sexual sadism, transvestic fetishism, voyeurism, and a final category of “other” paraphilias). It’s important to understand that these definitions can be very different from the way they are used in the media and sex-positive communities.
The most important distinction to make is that in most cases someone will be diagnosed with a paraphilia only if it’s a problem for the individual or someone in a sexual relationship with that individual. So having a foot fetish would not be enough to be diagnosed with a paraphilia, unless the fetish was causing significant distress or getting in the way of you living your life.
Paraphilias may be something that an individual always needs for sexual arousal, or they may be something that is required only at certain times (for example when the person is under a lot of stress).
There is very little data regarding how common paraphilias are. It’s been argued that the large amount of paraphilic pornography indicates how common paraphilias are, but this is hardly a scientific argument.
We know so little about paraphilias because people don’t often volunteer for treatment, even if their behaviors are causing a problem for themselves. As a result most of the information we have about paraphilias comes from individual case studies.