APA Citation Style is frequently used in the social sciences and natural/physical sciences. The first style rules were published in 1929 and the American Psychological Association has produced six editions of the Publication Manual since then.
APA style uses Parenthetical In-Text Citations within the main text of your paper. The sources that you refer to are then compiled into a References List at the end of the paper.
One key feature of APA style is that it now includes DOIs (Digital Object Identifiers), unique codes that are increasingly assigned to published articles.
Chicago Citation Style is used most often in historical research, although other humanities and social science disciplines sometimes use it as well. The first edition of the Chicago Manual of Style was published in 1906 by the University of Chicago Press.
Chicago style actually offers two different options for in-text citations:
In both variations of Chicago, the Bibliography at the end of the paper should include complete citations for all of the sources you referenced in a paper, and may also include sources that you consulted but did not end up paraphrasing, summarizing, or quoting in the paper's text.
MLA Citation Style is most frequently used in the humanities (literature, languages, art). The MLA Handbook was first published by the Modern Language Association in 1951.
MLA style uses Parenthetical In-Text Citations throughout your paper, with authors and page numbers. There is also a Works Cited page at the end of the paper with full citations.