In MLA documentation style, you acknowledge your sources by keying brief parenthetical citations in your text to an alphabetical list of works that appears at the end of the paper. The parenthetical citation that concludes the following sentence is typical of MLA style.
Ancient writers attributed the invention of the monochord to Pythagoras, who lived in the sixth century BC (Marcus 197). The citation "(Marcuse 197)" tells readers that the information in the sentence was derived from page 197 of a work by an author named Marcuse. If readers want more information about this source, they can turn to the Works-Cited list, where, under the name Marcuse, they would find the following information. Marcuse, Sibyl. A Survey of Musical Instruments. New York: Harper, 1975. This entry states that the work's author is Sibyl Marcuse and its title is A Survey of Musical Instruments. The remaining information relates, in shortened form, that the work was published in New York City by Harper and Row In 1975.
You should give a citation for all direct quotations that are not well known. You should also cite all facts and opinions that are not common knowledge - even when you have put the facts or opinions in to your own words. Two kinds of facts or opinions come under the heading common knowledge: those facts everyone in our culture should know (George Washington was the first president of the United States) and those facts that are common knowledge in the field you are investigating. If you do not pay the most careful attention to the techniques of quoting and crediting sources you run the risk of being accused of plagiarism.