Whenever you borrow another person's words or ideas when writing an essay, you must give that person credit using an accepted system of documentation. Failure to do this correctly is considered plagiarism. Different disciplines use different systems of documentation. In general, English and the humanities uses the Modern Language Association or MLA system of documentation. However, the American Psychological Association or APA system is used in the social sciences. You should check with your instructor before beginning any essay.
For more information on each system, visit these sites:
Modern Language Association (MLA)
American Psychological Association (APA)
Chicago or Turabian Style
Council of Biological Editors (CBE)