History

In order to be a successful scholar, all sources you use must have a citation. This is for several reasons, including:

• to avoid plagiarism. Citations give credit to the original author and show you have done appropriate research.
• to help other researchers. Citations allow your readers to locate the articles you used, and allow them to read them as well.
• to give credit where credit is due. Citations show that you have read other works--especially scholarly ones-- and acknowledge those who have researched your topic before you.

Chicago is a documentation style that has been published by the Chicago University Press since 1906. This citation style incorporates rules of grammar and punctuation common in American English. Chicago style allows for two types of source citation: Notes & Bibliography or Author-Date. Choosing between the two often depends on subject matter and the nature of sources cited, as each system is favored by different groups of scholars.
(Source: Official Chicago Manual website)

The notes and bibliography style is preferred by many in literature, history, and the arts. This style presents bibliographic information in footnotes or endnotes, typically supplemented with a bibliography. If there is no bibliography the full citation should be given in the notes field. If there is a bibliography, information in the note can be brief but must match an entry in the bibliography. Brief notes typically include the author's last name, the title of the work (usually shortened if it's more than four words), and a page number if appropriate.  

The author-date style is preferred by those in the sciences 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.