A sexual fetish (or, more properly, a paraphilia) is getting turned on by something that isn't considered a normal turn-on by one's culture. Of course this means what is considered a fetish will vary, and puritanical cultures will even consider any form of arousal to be taboo. Anything that isn't a normal turn-on could be a fetish. That is the other reason not to go into a detailed description of what exactly a fetish is. In fiction, there are general clues to show what's being treated as a fetish—namely, how people react to someone being aroused by something. If people are either disgusted or uncomfortable talking about it, it's generally a fetish. If people are impressed, it's kinky, but still on the edge of normal. Handcuffs can easily slide between fetish and kinky this way.
It's still uncommon to see a fetish openly discussed in mainstream fiction, and rare to see those with a fetish not treated like weirdos.
For a partial list of recognized fetishes, see That Other Wiki.