1. Submit a paper you wrote for another class, in a different class, without the permission of the instructors in both classes.
2. Submit and/or use a friend's paper or powerpoint as your own.
3. Submit and/or use a Prezi or other presentation file that you didn't originally create.
4. Use images off the Web without permission or citation (unless under Creative Commons or other permissions)
5. Use art or movie or video clips without permission or citation (unless under Creative Commons or other permissions)
6. Use music, even as background, without permission (and so on....) --

then you may have committed plagiarism! Most of the time you can use a small portion of a work (article, movie, score, etc.) for educational purposes, if you cite the source. But using more than that is not proper without specific permission from the author or creator. This is especially true if you are broadcasting or publishing the final product in a public way, such as an online video or blog. Reusing your own work is a problem because it's not 'original' for the more recent class, and you won't have had the chance to learn anything new.

Using reputable sources which you cite in your paper does several things. It helps put your work in an intellectual context, it supports your argument(s), so you aren't depending solely on your own knowledge, and it should protect you from most forms of plagiarism. You must cite sources that you get ideas, information or quotations from; you should not list sources in your bibliography that you didn't actually use in your paper.

The action or practice of taking someone else's work, idea, etc., and passing it off as one's own; literary theft.

Oxford English Dictionary, Plagiarism. Accessed June 10, 2010. http://www.oed.com/view/Entry/144939?redirectedFrom=Plagiarism#eid

Clearly, one can plagiarize other types of intellectual content beside literary works. Art, images, drawings, computer art, movies, photos, videos, and other 'fixed' sources are subject to copyright protection.