Swinging, which also called partner swapping (and in the old days, before the TV show, wife swapping), refers to a sexual activity where people in committed relationships have sex with other people, usually also in committed relationships. Swinging has been the subject of documentaries, feature films, books, and at least one network TV series. In public, swinging is represented almost exclusively by middle class, heterosexual, married couples. This may or may not be the case behind closed doors, but the white married suburban couple remains the face of swinging in the public sphere.
Swinging usually takes place at private parties where couples arrive together, have sex with others, and then go home together. Unlike polyamory, swinging focuses almost exclusively on the sex (it has also been called "sports fucking" by some who engage in it), and couples usually agree to avoid any emotional connection or relationships with the people they have sex with. One of the main tenants of swinging is that the couple always goes home together.
It's hard to say how many people engage in swinging, however a 2010 study that looked at sexually transmitted infections among swingers found that about 12% of people who came to a free sexual health clinic in the Netherlands identified themselves as swingers when asked by the counselor.
The importance of the social aspects of swinging differs from individual to individual and community to community, but the fact that swinging is also known as "the lifestyle" demonstrates that, for some at least, swinging is about much more than casual sexual hook-ups.