Citation is how you give credit to the people whose words, ideas or images you have used in your work. By citing sources, you create a trail for others to follow from your writing to your sources.
Photo: Abel & Co.1919; American Library Association and Library of Congress
There are three ways to use information from someone else's, or your own, work responsibly and ethically in your own work. In all three cases, you must give credit to the creator of that information by citing their work.
Quoting – Quote by copying the exact words from a source into your paper, and putting quotation marks around them. You quote when the words matter as much as or more than the ideas or information in a passage.
Paraphrasing – Paraphrase by rephrasing a sentence or short passage in your own words. It's not enough to just change the author's words -- you should completely change the way the information is expressed. You paraphrase when it's the ideas or information you need to express, and not the exact words.
Summarizing – Summarize by stating the main ideas of a source or section of a source in your own words. You summarize when you want to refer to a long section of a source or to present an overview of one of your source's ideas.
Always include a citation, whether you quote, paraphrase, or summarize.
If you quote, paraphrase or summarize your own writing (from another paper) you must cite yourself. "Recycling" part of or an entire paper from one class for another is considered plagiarism.
Check out "Quoting, Paraphrasing and Summarizing" at Purdue Owl.