Zotero is a (free) bibliographic management system -- it keeps track of references to books, articles, and other items that you want to use in your research. You can find out more about Zotero on their Web site.
The Chicago Manual of Style presents two basic documentation systems, the Humanities style (notes and bibliography) and the Author-Date system. Choosing between the two often depends on subject matter and nature of sources cited, as each system is favored by different groups of scholars.
The Humanities style is preferred by many in literature, history, and the arts. This style presents bibliographic information in notes and, often, a bibliography. It accommodates a variety of sources, including esoteric ones less appropriate to the author-date system.
The more concise Author-Date system has long been used by those in the physical, natural, and social sciences. In this system, sources are briefly cited in the text, usually in parentheses, by author’s last name and date of publication. The short citations are amplified in a list of references, where full bibliographic information is provided.