Swingers can be couples (married or not) or singles.
They have a sexually free spirit. Couples tend to be in love with each other, and only "share" each other sexually with others. Sometimes this means "swapping wives" sometimes it means "group sex".
Single swingers generally despise the dating process and would rather meet at swinger gatherings where it is acceptable and in some cases expected that sex can/will occur on the spot with whomever they meet and feel attracted to.
Tom and Jane go to a party where they meet Gary and Nancy. They go to a room as a group for sex.
Tom and Jane go to a party where they meet Gary and Nancy. They go to seperate rooms: Tom takes Nancy and Gary takes Jane for sex.
Melissa goes to a swinger party by herself. Melissa meets John and Tracy (a couple) at the party. melissa, John and Tracy all end up in a room together for sex.