BDSM is a three-way acronym that explains itself: 'BD' for bondage and discipline, 'DS' for dominant/submissive, and 'SM' for sadomasochism. BDSM covers an array of sexual kinks involving any combination of the three topics above, ranging from mild name-calling and emphasized dominant/submissive roles, to the traditional fetish for restraints and/or pain, and even further. BDSM in a fic is not in itself a charge; writing it badly can be, but one must bear in mind Your Kink Is Not My Kink.
It is very important to note that the presence of BDSM in a relationship is not an indicator of the state of that relationship. Just because a couple likes to participate in spankings doesn't mean they are actually abusing each other. Bad relationships can still happen, of course, but what happens during a BDSM session is (at least supposed to be) role-play or pretend-play meant to engage and arouse, and may not reflect what either partner wants or how they think in their public lives. A very independent and outgoing person can have the desire to be tied up and called nasty names; a pacifist can enjoy tying up and paddling someone. They're sexual fantasies, not social values.
To prevent real damage, physical or otherwise, many BDSM couples use or are encouraged to use a 'safe word'. Using this safe word means that the other partner will stop all activities. Regardless of what many may think, the unofficial big rule in most BDSM communities is the rule of Safe, Sane, and Consensual, meaning that a kink experience should be all of these things or not happen at all.
Unfortunately, it seems like some people didn't get the memo: BDSM is often used to mean rape/non-con by unwise fans who don't know or understand the details of the subculture or even what BDSM as a term means. This can be traumatic for unsuspecting readers, especially if there isn't an actual rape/non-con warning on there as well.