BookSurge is an Amazon subsidiary that provides print-on-demand (POD) service. POD is used by a variety of publishers who prefer to print certain titles in small batches, rather than ordering a large quantity of traditionally printed books, requiring immediate capital, shipping and storage.
BookSurge is marketed as a service for self-publishing authors, but is just one of several options. For a full discussion of the merits of POD versus offset printing--and self-publishing versus trade publishing--read _ Print-on-Demand Book Publishing_ by Morris Rosenthal.