Any idea or quote you use in writing your paper, that comes from other people or resources, needs to be cited. Citations will credit the original people and provide your reader with the information needed to identify your source.
A citation of a book generally includes: author(s), title, publisher, date. A citation of an article generally includes: author(s), article title, journal title, volume, pages, and date. Citations for web documents and articles from databases also include a URL and the date the information was accessed.
There are many different formats used in writing papers. The American Psychological Association (APA) and Modern Language Association (MLA) are two of the most popular. Always check with your professor to know which format or style should be used.
When writing a research paper, you will need to create a works cited list (or reference page) of the books, articles, websites you used. There are several different professional styles for doing just that - you will need to confirm with your instructor which one you need to use for a particular paper. The most prominent are: