BISAC subject headings are the standard book categories used in the U.S. book industry. (The UK uses BIC, France uses CLIL, Germany uses WGS, and so on.) The categories are updated regularly as new genres become popular. If you publish through Amazon, Barnes & Noble, or any other online retailer, you have probably come into contact with BISAC categories.
BISAC categories are particularly important for your e-books. Add as many as are appropriate for your book directly to the metadata inside the EPUB file.
The list of current BISAC headings is maintained by the Book Industry Study Group:
A global system for categorizing books, Thema, will come into use in 2014. Thema will work alongside BISAC and the other systems. If you sell internationally, it might be good to learn more about it so you can start including the Thema categories in books.