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.
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- to 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.
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.