Always check with your professor about the preferred style. The most common are:
literature, languages, other humanities
psychology, social sciences, business
history, religion and theology, engineering, and some sciences
Different conventions have developed within different fields, in isolation from each other. It is so rare that a scholar in one field (say, zoology) reads scholarly articles in an unrelated field (say, moral philosophy) that citation conventions tend to grow more dissimilar, like finches on the Galapagos Islands.
What we have provided below is a brief guide to the hallmarks of formatting within each of the major styles, and explanations about why some of those formatting decisions actually matter. For detailed instructions on formatting, follow the links to style guides.
1. The citation itself. This can take the form of a superscript footnote number 1, a number in brackets [1], a name and date in parentheses (Smith 2008) or a name and page number in parentheses (Smith 47) or perhaps a name in text, like Smith, and a page number in parentheses (47), depending on the citation style. As you get used to reading for citations, your eyes will seek out superscript, brackets, parentheses, and numbers. An APA parenthetical citation might look something like this:
Willis & Schor (2012), however, provide evidence that consumer behaviors correlate with political action.
2. The full bibliographic entry. In most citation styles, this appears at the end of the article or chapter, or perhaps even at the end of the book. An APA bibliographic entry for an article looks something like this:
Willis, M. M., & Schor, J. B. (2012). Does changing a light bulb lead to changing the world? Political action and the conscious consumer. Annals of the American Academy of Political and Social Science, (1), 160-190. doi:http://dx.doi.org/10.1177/0002716212454831
MLA Handbook for Writers of Research Papers by 
MLA Style Manual and Guide to Scholarly Publishing by 
Identifying the author and precise page (or line number, for verse or sacred texts) where quoted material is located is more important than currency of research.
(Freier 7)
Because being "up to date" isn't a big concern in the humanities (go ahead and cite Aristotle), publication year is absent from in-text and parenthetical citations and less prominent in bibliographies.
(Freier 8-9)
Bibliographic entries identify the precise version of a text, because there are often variants.
Author names are often located outside parenthetical citations. When a source's author is the subject or object of a sentence, it's much easier to show how that author participates in the scholarly conversation. This is also a way to give a "shout out" to an accomplished colleague.
(Freier 6)
Publication Manual of the American Psychological Association by 
Establishing authority and currency of cited research is important, so the researchers' names and the year are prominent in both in-text citations and bibliographic entries.
Author/researcher names are also important markers of authority and schools of thought.
Last names + first initials avoid gender bias.
(Van Deth, 2014)
Chicago Style (aka "Turabian"*) is unusual in that it is used by fields in the humanities, social sciences, and sciences. Multiple methods in source-use have led to two distinct styles within Chicago:
1. Author-Date This style is adapted to research more similar to social sciences, in which the author's name and the currency of research are both important and need to be displayed in text. Parenthetical citations are similar to APA.
2. Notes-Bibliography This style is adapted to research in the humanities in which frequent parenthetical interruptions would interfere with a reader's smooth comprehension of the text. Historians often use this style, offering readers a chance to skip past frequent citations, and only consult footnotes if there is a question.
1. Gloria Leckie, “Desperately Seeking Citations: Uncovering Faculty Assumptions about the Undergraduate Research Process,” Journal of Academic Librarianship 22, no. 3 (1996): 201-208.
2.Cullen Chandler and Alison Gregory, “Sleeping with the Enemy: Wikipedia in the College Classroom,” The History Teacher 43, no. 2 (2010): 247-257.
3. Barbara Fister, “The Research Processes of Undergraduate Students,” Journal of Academic Librarianship 18, no. 3 (1992): 163-169.
*Though people often use "Chicago" and "Turabian" interchangeably, there are differences between the two. "Chicago" was developed for publications by the University of Chicago Press. "Turabian" was developed at the University of Chicago by Kate L. Turabian for dissertations at the university. The 16th Edition of The Chicago Manual of Style has begun to fuse the two, and the University of Chicago has actually purchased the rights to the "Turabian" name and system, so differences may soon disappear.
In the sciences, researchers often use the citation style of the leading journal in their area instead of following a standard style guide published by their professional organization. Check the syllabus or consult with your instructor regarding the preferred style for the assignment. A librarian can help you find the guidelines for each style.
For science assignments that require a professional organization's style guide:
Legal citation is tricky, and best learned in law school. The most frequently used style manual for citing to legal documents is The Bluebook: a uniform system of citation. APA, MLA and Chicago Manual of Style all refer to the Bluebook for citing to certain documents such as cases. More Legal Citation Help
The Bluebook by 
Citation is a tricky balance between brevity and precision. Each citation must refer to a single entry in the bibliography, which in turn must refer to a single source. But each citation must also be as brief as possible, so it doesn't interfere with smooth reading. To achieve both brevity and precision, styles employ abbreviations, punctuation, and typography to indicate types of information. Note the space saved by using an APA style citation, compared to how a novice writer might cite:
All of that information gets pushed to the back of the article, to the bibliography. As you get used to the conventions, you will start to refer to articles by an author's last name, not by a title. That's what scholars do. Note: Ultimately, precision wins. When in doubt, be thorough.
