Short answers, pulled from the story.
BDSM stands for Bondage and Discipline, Dominance and submission, and Sadism and Masochism. The initialism first appeared in a reply posted on the 20th of June 1991 by an anonymous user in the Usenet newsgroup alt.sex.bondage, and was reposted in other forums between 1995 and 1997.
Safe, sane and consensual, abbreviated SSC, is a code of conduct adopted by many BDSM practitioners and organizations since the 1980s. It originated in the statement of purpose of GMSMA, a gay SM activist organization, and requires that activities be physically safe, that all participants are of sound mind to consent, and that all participants do consent.
Estimates vary across studies and countries. A representative Australian study from 2001-2002 found that 1.8 percent of sexually active adults had engaged in BDSM in the previous year. A 2017 Belgian survey found 12.5 percent of the population practiced BDSM on a regular basis. Researchers broadly estimate that between 5 and 25 percent of the population engages in behavior related to pain or dominance and submission.
Denmark was the first European Union country to remove sadomasochism from its national disease classification in 1995, followed by Sweden in 2009, Norway in 2010, Finland in 2011, and Iceland in 2015. The World Health Organization removed sadomasochism from the ICD-11, published on the 18th of June 2018.
The Folsom Street Fair, held annually in San Francisco, is the world's largest BDSM event. It has its roots in the gay leather movement and takes place in a public, clothing-optional space between 8th and 13th streets, with nightly parties associated with the event.
A 2008 Australian study by Richters and colleagues found that BDSM practitioners were not more likely than the general population to have experienced sexual assault or to feel unhappy or anxious, and that BDSM males reported higher levels of psychological well-being than controls. Psychotherapist Charles Moser has stated there is no evidence that BDSM practitioners have any common psychopathology or commit violent crimes at higher rates.