What is a margin account?

What is a margin account?
Research margin accounts and write a 2-3 page paper (not including the cover page or a reference page) on:

        What is a margin account?

        Why would investors use one?

        What are the possible downsides to buying/selling securities on margin?

        A minimum of two properly cited references are required, with at least one being from an outside source you find by searching the Internet.