Smaller venues, including pub theatres, are generally called Fringe Theatres (sometimes referred to as 'Off West End' by our American cousins!) These theatres can vary in size, with seating capacities starting from around 40 up to as many as 400.
You can see some superb plays in Fringe theatres, and ticket prices are generally speaking much lower than for West End productions. The sheer intimacy of Fringe theatres brings you up very close and personal to the action provides a completely different experience to the larger West End venues.
There are also a few other commercial theatres in London that are not considered either West End or Fringe such as the Richmond and Wimbledon theatres. Due to their locations in the London suburbs these venues are classed as regional and mainly receive touring productions, which only play for a week or two.