BDSM is an acronym formed by the merger of two separate categories of sexual practice: Bondage and Discipline, and Sadist-Masochist.
As a catchall term, BDSM is used today (2016) to refer to any sexual, or quasi-sexual, activity in which there is a voluntary power imbalance. As a result of prior negotiation, one partner agrees to having pain inflicted by the other, or to be immobilized, or to be blindfolded and made to submit to anonymous sex, or to have their life micromanaged. As a result of the mainstreaming of fetishes of all sorts, BDSM, even if not called by that name, is often found at sex clubs of all sorts. Commercial "dungeons" now exist in many cities to provide an environment for BDSM to be practiced and enjoyed. In the peculiar dichotomy found in the sexually conflicted, yet progressive United States, in which causing another (consensual) pain is fine, but pleasure is not, public BDSM play spaces prohibit overt sexuality, or sometimes even nudity. Paying someone to whip you is fine, paying someone to pleasure you sexually is not.
Any activity in which one is immobilized is potentially hazardous. Commercial dungeons typically have a "dungeon master", who does not participate but supervises with an eye towards safety.
The participation of boys in BDSM is never found in a public or semi-public setting. Unfortunately, in some parts of the world, usually under dictatorial or corrupt governments, non-consensual BDSM — actual torture — is not rare, according to those cases which become public.