Book Industry Standards and Communications code, technically the BISAC Subject Heading List. An industry-approved list of subject descriptors, each represented by a nine-character alphanumeric code. The descriptor itself consists of two, three, or four parts. For example, the code for the descriptor representing general African history is HIS001000 and the related descriptor is "HISTORY/Africa/General."
There are 50 major sections; within each one are several detailed descriptors that represent subtopics deemed most appropriate for the major topic. The Subject Headings were developed to standardize the electronic transfer of information between trading partners, as search terms in major bibliographic databases, as access points for database searching, and as shelving guides.
You can sometimes see BISAC codes on books themselves--check the back cover. :-) Also, see category ghetto for an interesting twist on using (or not using) these codes to shelve books in particular sections.