The Chicago Manual of Style uses two different documentation systems: Notes & Bibiography and Author-Date.
In the notes & bibliography system each source is cited in the text by a superscript (raised) number in the text. The number refers to a citation that is included in a footnote (at the bottom of the page) or an endnote (at the end of the text). The sources used for the paper will also probably be listed in a bibliography. Notes and bibliography is frequently used in history, literature, and the arts.
In the author-date system each source is cited in the text by the author's last name and the year of publication (usually in parentheses). The full citation for the source is given in a list of references. The author-date system is frequently used in the social sciences and sciences.