In BDSM terminology, a party designed to foster semi-public play. And a chance to get dressed up (or down), without the pesky anti-nudity laws applied to most goth/fetish clubs.
Play parties are normally held in private homes/dungeons or hotel rooms, and by invitation only. This can range from casually having friends over for coffee and "dessert" to a 50-person affair in a dungeon devoted to that purpose, complete with permanent equipment and ceiling fixtures. Some BDSM and sex clubs will rent their space to very large regional groups for special parties. Usually there is some cover charge to take care of the cost of rent and utilities on the place, as well as any food or beverages that might be provided.
In many locations assault can be prosecuted, even when both parties have consented to the activity. For confidentiality reasons, parties are often "closed list," and new additions to the list or guests must sign a statement saying that they will not disclose the location, nature or identies of people present at said party.
While the main purpose of these parties is to actually play, there is some down-time for conversation when all the good toys are being used or people are just exhausted. Although BDSM activity is normally erotic in nature and closely related to sex, in my experience of parties in the North East US, public sex is not the focus, and is sometimes restricted to designated "sex-friendly" rooms.
The atmosphere is usually dim or gently lit, with seating and some scattered bondage furniture. You can expect industrial/goth type music, and maybe some tasteless porn playing on a screen somewhere. Private parties don't usually have a dress code to speak of, other than that at the end of the night, you leave clothed, so as not to draw the attention of the neighbors. You are almost guaranteed to meet: a woman in a corset, a collared submissive and his/her Dom/me, the creepy guy who just watches, and lots of people wearing black.
A play party offers an opportunity to observe others while they scene, show off your single-tail skills/new rope kit/flogger collection, and to meet up with people that you would like to develop deeper relationships with. Play parties can be considered slightly safer than private play, assuming you trust the host, because there are many more sets of eyes monitoring your scene at any given time. This makes them ideal for playing with someone new (feeling them out) before you are ready to either enter their home or invite them into yours. They also provide an opportunity for people who cannot safely practice BDSM in their own homes.
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