There are generally three categories of art galleries: commercial art galleries, public art galleries, and parallel galleries.
Commercial galleries are the homes of art dealers, and getting into one is like being baptized: to have a major dealer sell your work is 90% of the battle to become a successful, rather than a starving artist.
Public galleries don't sell, and are supported on donations and government money. They are never the front line in art, because commercial galleries get the successful artists first. Public galleries make due with retrospectives and commercially unsuccessful artists who survive on grants, teaching positions, and day jobs.
Parallel galleries are the indie scene in art. Very often poor and supported by small donations and small government grants, parallel galleries explicitly offer a venue to unrecognized artists, and occasionally try to provide a commercial space for those artists to sell something. They usually exist in vacant spaces that are cheap to rent, and some move every few months as they can organize a show. Often, parallel galleries are where art students cut their teeth in organizing a show and displaying their work. Parallel galleries are often staffed by the same artists who show there, who volunteer to get experience and exposure.