Bibliographic works differ in the amount of detail depending on the purpose and can generally be divided into two categories: enumerative bibliography (also called compilative, reference or systematic), which results in an overview of publications in a particular category and analytical or critical bibliography, which studies the production of books. In earlier times, bibliography mostly focused on books. Now, both categories of bibliography cover works in other media including audio recordings, motion pictures and videos, graphic objects, databases, CD-ROMs and websites.