It is a party mainly for people to get drunk and have sex. Usually random people show up and the party is often kept quiet from the public. Many people who are in relationships show up at swinger parties to 'swing' or enjoy a different style of sex. Orgies and group sex is very common at swing parties too.
Guy: I went to a swingers party to drink and fuck random girls that I'll never see again in my life.
Girl: Let's go to the swingers party across the road and have 12in cocks ploughed into our tight cherry pussies.