Discuss differences in the source formats: MLA, Chicago, etx.
The major difference in the formatting of citations has to do with relevancy of the date of publication. The Modern Language Association's (MLA) formatting style is used primarily in the humanities and the arts. The format leaves the date of publication until later in the citation and focuses on the author and title of the work. Often scholars who write in these fields use texts with early publication dates or they may refer to esoteric texts that are noted by the author, not the date. If someone writes a critical essay on Hamlet and uses a secondary source as support, does it really matter if the source is from 1920 or 2010? Not really (minor exceptions of course apply!).
On the other hand, the American Psychological Association's (APA) formatting calls for immediate attention to author and date. This type of formatting is used typically in the sciences because the recent nature of the research is more important in these fields. If someone is writing about the planets in our solar system, sources from 1980 will not be appropriate or accurate.
Diana Hacker provides detailed information about formatting types in her reference books and on her website with Bedford/St. Martin's Press. The link is provided below. In addition, there is a link to the Chicago Manual of Style which further explains source documentation.