A comedy club is a venue, typically a nightclub, bar, or restaurant where people watch or listen to performances, including stand-up comedians, improvisational comedians, impersonators, magicians, ventriloquists and other comedy acts. The term "comedy club" usually refers to venues that feature standup comedy, as distinguished from improv theaters that host improv or sketch comedy and variety clubs that may also host musical acts.
Comedy clubs are usually broken down by comedians into "A rooms", "B rooms" and "C rooms"
A rooms usually cater to people with movie deals, people with television shows, and generally well known acts.
C rooms act as "neighborhood" comedy clubs, for the most part. The headliners are not usually very well known or popular, and the audiences are random walk-ins.
B rooms are where the best aspects of both A rooms and C rooms meet. Young comics need B rooms as a stepping stone. These are rooms where someone doing a 10-15 minute set (hosting/MCing) can be asked, after they've been going up long enough, to do a 20 minute set (featuring) and so on. These clubs also typically allow dirtier material, since they can become established names for "dirty" comedy or shows that usually cover adult themes.
For an example: "The Comedy Zone" Comedy Clubs have A, B & C rooms. They are the largest comedy booking agency in the world.