Kink is the exploration of sex outside of vanilla sex. This incldes BDSM, power exchanges, fetishism, and dominance/submission.
Some fetishes can be directly attributed to a psychological problem, sometimes stemming from childhood abuse. A kink-aware psychologist can determine if a person's fetish is simply that or a result of some kind of trauma or conditioning. That's a very important thing to know considering oftentimes a fetish will be taken too far in the need to satisfy the need for it as a coping method, putting the person in physical and/or mental harm.
And sometimes, people just happen to like being ****** up the ass wearing nipple clamps and a dog hood.
I knew a man who liked voyeruism, scat, watersports, and anything messy. He grew up as "normal" as any other kid, but simply developed a taste for that kind of play. Personally not my thing, but I have no room to talk.