As originally practised by British aristocrats, a party taking place over a period of days, rather than simply a afternoon or evening, with the participants sleeping over in guest rooms and eating, drinking, and socializing in common, often in a manor house or other Stately Home.
This suggests, to a modern ear, a rather pleasant state of anarchy, with everyone coming, going, and availing themselves at will of the swimming pool, golf course, etc. The truth is that most of these events were as rigidly scheduled as a day in High School or a Holiday Camp (wakey, wakey!), with everyone being shuttled around en masse from one activity to another (some thoughtful hosts provided printed schedules on arrival), usually centered on say, hunting, a local cricket match, or suchlike seasonal activity. Those who chose not to participate in, say, tennis or riding would be given someplace to sit (and usually some kind of refreshment as well), or less strenuous activity in the same general area, but a gracious guest was expected to at least try their hand at whatever was being offered. (The concept of social accomplishments helped a great deal in this: most of the people there had more-or-less the same skill set, and so activities became a lot more easy to plan.) In case of inclement weather, activity would be curtailed to walking (in the garden) or a motoring excursion (to a church, pub, or other local attraction), but on no account were people allowed to simply remain in the house itself. This rigidity was due to two binding constraints, one concerning servants, and the other implicit in the country home lifestyle itself.
First, even the richest families with the most devoted servants were at the mercy of law and custom regarding who worked where and for how long: since every part of the house had to be cleaned on a regular basis, and quick meals almost impossible, it simply wouldn't do to have, say, a few male guests getting up at the crack of dawn for a quick plunge, while some ladies lay in bed until noon or later. Also, while we love to picture a Lady of the Manor striding through a house full of displayed antiques, with every room awaiting only a human presence, very little of a manor house was in use at one time: to preserve what grandeur there was, much of the house was locked up, under dust covers, or simply kept bare until the few hours it would have to be on display.
House parties were most in vogue in the late 19th and early 20th centuries, latterly in the form of "weekends", forming a ready background for novelists trying to portray a range of philosophies-in-collision, such as Aldous Huxley's Crome Yellow (or a nice little murder mystery!) In the 60's and 70's they became popular again, with a lot less regimentation, in town this time, among the young and hip: see apt festival.