A quotation is the repetition of one expression as part of another one, particularly when the quoted expression is well-known or explicitly attributed by citation to its original source, and it is indicated by (punctuated with) quotation marks.
A quotation can also refer to the repeated use of units of any other form of expression, especially parts of artistic works: elements of a painting, scenes from a movie or sections from a musical composition.
Reasons for using quotations
Quotations are used for a variety of reasons: to illuminate the meaning or to support the arguments of the work in which it is being quoted, to provide direct information about the work being quoted (whether in order to discuss it, positively or negatively), to pay homage to the original work or author, to make the user of the quotation seem well-read, and/or to comply with copyright law. Quotations are also commonly printed as a means of inspiration and to invoke philosophical thoughts from the reader. Pragmatically speaking, quotations can also be used as language games (in the Wittgensteinian sense of the term) to manipulate social order and the structure of society. 
Common quotation sources
Famous quotations are frequently collected in books that are sometimes called quotation dictionaries or treasuries. Of these, Bartlett's Familiar Quotations, The Oxford Dictionary of Quotations, The Columbia Dictionary of Quotations, The Yale Book of Quotations and The Macmillan Book of Proverbs, Maxims, and Famous Phrases are considered among the most reliable and comprehensive sources. Diaries and calendars often include quotations for entertainment or inspirational purposes, and small, dedicated sections in newspapers and weekly magazines—with recent quotations by leading personalities on current topics—have also become commonplace.
Quotations and the Internet
Chiefly a text medium in the beginning, the World Wide Web gave rise to any number of personal quotation collections that continue to flourish, even though very few of them seem to facilitate accurate information or correct citation.
The increase of written means of informal communication brought about by the Internet has produced the practice of using quotations as personal flags, as in one's own signature block. This is most commonly seen in email messages and Usenet posts, while it is almost never seen in blog posts. Quotations are also popular as a user's personal message, a line under the user's nickname in some Instant Messaging clients (and here they often go uncited). In all these cases, quotations are usually included to give a glimpse of the user's personality, to make a statement of their beliefs, or to spread views and ideas.
The sheer bulk of online quotations, combined with more efficient search engines, has effectively made the Internet the world's quotation storehouse, encompassing an unprecedented number of easily obtainable quotations. Though matters of accuracy still remain, features such as Amazon.com's Search Inside the Book and Google Book Search may serve to alleviate such concerns.
United Kingdom copyright law
Section 30(1) of the United Kingdom Copyright, Designs and Patents Act 1988 (apparently in transposition of Article 5(3)(d) of the EU Copyright Directive on quotations) allows fair dealing with a copyrighted work for the purpose of criticism or review, provided that it is accompanied by sufficient acknowledgement.
Many quotations are routinely incorrect or attributed to the wrong authors, and quotations from obscure or unknown writers are often attributed to far more famous writers. Examples of this are Winston Churchill, to whom many political quotations of uncertain origin are attributed, and Oscar Wilde, to whom anonymous humorous quotations are sometimes attributed.
The Star Trek catchphrase "Beam me up, Scotty" did not appear in that form in the original series. Other misquotations include "Just the facts, ma'am" (attributed to Jack Webb's character of Joe Friday on Dragnet), "Heavy Lies the Crown" from Shakespeare's Play Henry IV, Part 2, "Elementary, my dear Watson" (attributed to Sherlock Holmes), "Luke, I am your father" (attributed to Darth Vader in Star Wars), "Play it again, Sam" (attributed to Ilsa in Casablanca), "Do you feel lucky, punk?" (attributed to Harry Callahan in Dirty Harry) and "We don't need no stinkin' badges!" (attributed to Gold Hat in The Treasure of the Sierra Madre). 
The Dirty Harry quotation "Do you feel lucky, punk?" is a rewording of the original dialogue: "You've got to ask yourself one question: 'Do I feel lucky?' Well, do ya, punk?" Humphrey Bogart's character Rick in Casablanca never said "Play it again, Sam." The actual expression is "Play it, Sam." Mae West did not say "Why don't you come up sometime and see me?" in She Done Him Wrong, but rather "Why don't you come up and see me sometime?"
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- "English Grammar Lesson - Using Quotes! - ELC". ELC - English Language Center. 2016-11-16. Retrieved 2017-10-24.
- Capone, A., & Salmani Nodoushan, M. A. (2014). On indirect reports and language games: Evidence from Persian. Rivista Italiana di Filosofia del Linguaggio, 8(2), 26-42.
- Salmani Nodoushan, M. A. (2015). The secret life of slurs from the perspective of reported speech. Rivista Italiana di Filosofia del Linguaggio, 9(2), 92-112.
- See A Book of Misquotations, edited by Elizabeth Knowles, Oxford University Press, 2006.
- The Holmes phrase originated in a radio play. See List of misquotations and "Elementary, My Dear Watson" at Snopes.com
- Webb did say: "All we want are the facts ma'am". See Just the facts, ma'am, List of misquotations and "Just the Facts" at Snopes.com
- Greatest Film Misquotes - Part 2, Tim Dirks at filmsite.org
- on YouTube although the last of these is spoken by one of the Mexican Bandits that Hedley Lamarr attempts to hire as mercenaries in Blazing Saddles