Chicago Manual of Style gives writers two different methods for documenting sources: the Author-Date System and the Notes-Bibliography (NB) System. As its name suggests, Author-Date uses parenthetical citations in the text to reference the source's author's last name and the year of publication. Each parenthetical citation corresponds to an entry on a References page that concludes the document.
By contrast, NB uses numbered footnotes in the text to direct the reader to a shortened citation at the bottom of the page. This corresponds to a fuller citation on a Bibliography page that concludes the document. Though the general principles of citation are the same here, the citations themselves are formatted differently from the way they appear in Author-Date.
Make sure you are using the same citation methods throughout your research assignments.