Houses are organisations which allow every member to speak to every other member who is currently playing at once. This is both a blessing - allowing for co-ordinated action, and a curse - preventing anyone from sleeping. In essense they are just another communications options, though in practise the choice of which house a player belongs to becomes part of their identity. Who they associate with is who they are. Players can join a house when they have advanced a couple of levels, though many houses let players join only through invitation - a device employed to keep out the killers and the inexperienced. House leaders and captains can admit and expell players at will, which gives them a certain power over their members - though one which they dare not exercise too freely.
On the downside, thirty members is not quite enough to provide for an effective force of players on at any one time. A mass co-ordinated anti-pk hunt cannot be managed in quite the same manner - though to be fair this is much more a function of the skills and attitudes of the players (their faith in their skills and their trust on each other to provide backup in emergency), than pure numbers.
But I'm just being nostalgic for those fun times when hordes of terribly bad players, led by a few reasonable ones pursued one or two excellent killers and either slew them - or by some miraculous incompetance were themselves all forced to flee in terror. These days tearoom sitting and individual daring have again come back into popularity.
Of the other recently large houses, MADCAP (I think) was destroyed by its leader and IC was destroyed through corruption. IC was pretty much doomed anyway as Rhye had become its leader - and he was unable to connect to wireplay, so it was destined not to thrive with no way of replacing its fallen captains.
The total list of houses has continued to lengthen over time. There are now perhaps fifteen in total, many of which have fewer and three members. The number of sizable houses is currently ONE. AOF hovers between twentyfive and thirty, beneath that there is a massive drop to a couple of houses with around ten, and then the long tail of tiny houses.
Until very recently there were THREE houses with over twenty members each, so there is plenty of potential for someone with the fight attitude to build a strong new house - think of all those houseless personae out there!