Medically a "Fetish" is the eroticization of a physical object that is not normally considered sexual, such as a shoe or a specific body part. When we eroticize an act or situation, as with spanking or being bound, it's called a "paraphilia". In either case it's only considered a problem when it interferes with one's ability to function in a normal social context like holding a job. The idea of a "fetish" being distinct from a matter of taste dates back to Freud and Kraft-Ebbing who defined any sexual tastes other than those approved by Victorian morality as a form of mental illness. In practical terms today, the only real difference between liking something and having a fetish is that a fetish is understood to be sexual in nature.