Swinging is a form of recreational social sex between consenting adults, most commonly consisting of male/female couples meeting other male/female couples for sex and/or ongoing intimate friendships. In its simplest form, "swinging" is enjoying consensual sexual experiences with people other than your regular partner.
Most people, who swing and are part of a couple, do so with the full knowledge, consent and, usually the participation of, their partner. Within this, however, there is a broad spectrum of activities. For some, swinging forms a big part of their life, for others it is an occasional "treat". Some people enjoy a wide variety of swinging partners; others form long-standing relationships with one couple. For some, swingers "anything goes", while others confine themselves to limited experiences that they enjoy, for example girls exploring their bisexuality together whilst the partners simply watch.
Swingers have traditionally been largely middle class and tend to blend in quite easily with the general population in terms of appearance and ideology.