By definition, a chamber of commerce is a voluntary association of people whose function is to promote and develop the commercial and industrial opportunities in a community. The association's members generally consist of businesspeople. A chamber collectively acts on behalf of its members, much like a spokesperson, communicating their desires for the betterment of the community to parties in power that can see them to fruition.
Chambers of commerce aren't limited to cities. There's an International Chamber of Commerce. They also exist in towns, regions, states and nations; the United States has an official chamber known as the U.S. Chamber of Commerce that represents more than three million businesses. Local chambers don't answer to the U.S. Chamber; these are autonomous entities. However, they may choose to work with the U.S. Chamber and allow it to be their mouthpiece if they have the same goals in mind, for example, to sponsor a bill.
Minority chambers like the San Francisco African American Chamber of Commerce ensure the specific needs of their constituencies, such as business owners, are being heard and met throughout the community. The National Gay and Lesbian Chamber of Commerce's purpose, or function, is to expand the economic opportunities of the lesbian, gay, bisexual and transgender community.