Actually it can be any venue but a pub is common. The usual format is that there are resident singers who perform most weeks as warm up,fill-in etc. Guests who are booked and are well known people in the folk world, and sometimes singers nights where anyone can ask for a spot. A club normally rotates the activities. A club that offers only floor spots is often known as an Open Mic club.
Entrance is by payment on the door although for big draw guests it is sometimes tickets in advance. Some clubs have membership, some don't. There is usually an organising committee of some sort to book acts and arrange the schedules.
Some well known folk clubs include: