MLA, which stands for Modern Language Association, is simply a standardized way to write, format and cite your sources. When you quote outside sources in MLA format, they must be cited or else you are committing plagiarism. Many of your professors, especially your English and Humanities professors, will expect you to write in MLA format, which means that you are citing sources that have a very specific way of looking on the page.
In order to write in MLA format, you must have two things: 1. in-text citations and 2. a Works Cited page.
The in-text citations tell the reader who you are quoting and what page or paragraph you are quoting from. Then the Works Cited page tells your reader how to find that book or magazine or newspaper etc. that you are quoting. As you can see here, the sources/authors cited in the paper are the same sources that are in the Works Cited page which is a requirement at the end of the paper.