Citations document information sources used in academic research, and they serve as an "address" of sorts to those sources.  Regardless of the format/style you're using (APA, MLA, SBL, AMS, ACS, CSE, or others), they usually take two forms within the same work:  Reference or complete citations, and shorter in-text or parenthetical citations (sometimes also called "author-date" citations) that correspond to the reference citations.

• Reference or complete citations, are the complete entries that are usually found gathered together in bibliographies, reference lists, and works cited or works consulted lists that usually appear at the back of books, at the end of articles and papers, and even on some web pages.  (They instead sometimes may appear individually in the form of "footnotes" at the bottom ["foot"] of relevant pages rather than together at the end of research in a single list.)  These full citations contain all the information necessary (an "address") for the reader to locate those sources to consult them for themselves (similar to how an address for a residence or business enables you to find its location).

• In-text citations (often called "parenthetical citations" because the citation appears within parentheses) occur in the research text narrative (such as the body of a paper, a book chapter, or an article) and are enclosed in parentheses.  These parenthetical citations are a kind of short-hand version of the full references located in references/works cited lists and refer the reader to the complete citation in the reference or works cited lists.  

Examples of reference citations and their corresponding in-text counterparts:

A citation for a book (using APA Style, 7th ed.):

• Reference/Complete Citation:  

Livermore, J. B., & Quigley, E. (2002). Field assessment in crisis counseling (2nd ed.). Sage.

• In-Text/Parenthetical Corresponding Citation:  

(Livermore & Quigley, 2002)  

 A citation for an article in print form (using MLA style, 8th ed.):

• Reference/Complete Citation:  

Piper, Andrew. "Rethinking the Print Object: Goethe and the Book of Everything." PMLA, vol. 121, no. 1,  Jan. 2006, pp.

     124-38.  

• In-Text/Parenthetical Corresponding Citation: 

(Piper 125)  

Usually, reference or complete citations that appear in References (APA style) or Works Cited (in MLA style) lists include at least several of the following elements:

For Books: 

• the author, editor, translator, or other person responsible for the content 
• the date of publication
• the title of the portion of the work being cited if relevant (for example, the book chapter or section, etc.)
• the title of the book in which the work being cited is published 
• the edition, if higher than the first (for example, revised edition, third edition, special edition, etc.)
• the publishing company's name 
• the place of publication 

​For Articles in Magazines, Journals, & Newspapers:

• the author(s) responsible for the content 
• the date of publication
• the title of the work being cited (for example, the article, etc.)
• the title of the journal/magazine/newspaper in which the work being cited is published 
• the volume and issue numbers corresponding to the date of publication 
• the page numbers on which the work appears
• some styles, such as APA, require a number unique to a specific journal article, called a DOI number
• sometimes, the URL for an article in an electronic database (depending on whether you're using APA or MLA style)
• sometimes the name of an electronic database, (depending on whether you're using APA or MLA style)
• sometimes the format (for example, print, web, etc.), depending on whether you're using APA or MLA style

APA and MLA Styles differ to some extent in what their in-text citations contain.  But in both styles, the in-text citations include briefer information and refer you to the reference citations for more complete information.  Every in-text citation must point to a more complete reference citation in a References or Works Cited page at the end--no "Lone Ranger" in-text citations are allowed!

• APA Style:  In general, in-text citations include the author's name (or names), year of publication, and when documenting a direct quote, page numbers on which the material is found, all separated by commas:  (Livermore & Quigley, 2002) or (Livermore & Quigley, 2002, p. 65)
• MLA Style:  In-text citations include the author's name (or names), and page number (or other part, such as a paragraph, section, etc.) on which the information is found, regardless of whether it's a direct quote or not.  No commas are used between the author's name and page number, and the date of publication is not included. (Piper 126)