When large publishing houses won't publish something because it's not 'mainstream' enough. Somebody decides to create their own small publishing house and self-publish. This process probably started shortly after the printing press was invented. But it was certainly accellerated by the invention of the copy machine. An example that a lot of people are familiar with is the ubiquitous zine. The most common form of zine is the fanzine. To see a more comprehensive view of the world of small press, see Alternative Press Review which lists and reviews hundreds of publications, like Factsheet Five used to do. www.altpr.org/
Some examples include: