Well, someone has to start, and bearing in mind this is a moderated permathread, this post may be replaced by more apposite ones later.
But, from personal experience.

It depends on size, size of venue, number of members/attendees.

It depends on whether there is a stated guest, or it is just an 'open mike' session where everyone who wants can get up & sing. Bearing in mind that these type sessions will not necessarily even possess a 'mike'.

Usually it is on licensed premises, in fact under the new laws, that seems a legal requirement. But I was referring to an alcohol license.
On licensed premises it will usually have a room of its own for the evening, with drink served in a separate room. This means less distractions for the singers, and (if the organisers wish it) the presence of a younger element to continue the traditions.
The premises will usually sell "Real Ale". This may confuse some, but the main difference between this and 'keg beers' or lagers is that the beer continues fermenting within the barrel, and so is not reliant on overpressurisation or added gas to serve. A constant need to belch (or worse) can be most off-putting to a singer.

Payment: there may be a charge 'on the door', or there may be a raffle (possibly for a bottle, or a tape/CD gifted by a previous guest). There may be both.
If there is a cover charge made at the door, this will often be increased on Guest nights, and possibly reduced for those who have shown some commitment to the club by buying an annual membership.

Annual memberships also allow some clubs to prepare a periodical newsletter (even if only one page of A4 folded) but this lets them improve the chance of getting enough people in to make guest nights successful.

Usually you will spot a table with adverts for other clubs, forthcoming events etc.,

Finally (to give someone else a chance to comment) Some clubs may have a recognised charity to which they contribute any profits. But most clubs seem to be non-profit making concerns. I somehow doubt though that this is a deliberate policy!

CHEERS
Nigel