Plagiarism is using another author’s work as if it were your own. This is not only cutting-and-pasting sentences from another source or buying old papers online! Instead, plagiarism includes borrowing someone’s words, information or ideas without giving proper credit.
Scholarship is based on making use of the work of published authors writing about your topic, but you must provide a complete citation. You must cite information, no matter where you find it. This is as true for material found online or on the wall of a museum as it is for tradition print sources.
If you use someone else’s words, word for word, put quotation marks around them and provide a footnote. This is great. If you do not do both of these, it is a problem. If you take information from a source, and do not provide a citation, this is also a problem.
Many students seem to think that if their paper is full of citations to the work of others, it will look as if they didn’t do anything, that they just strung the other sources together. This is quite backward! Pick up a copy of any academic journal (Art Bulletin, Art Journal, October, Gesta, Word and Image, etc), and you will see, in some cases, as much of the page taken up with footnotes as with the author’s main text! The more notes you have, the more research it is clear you did. I have never heard of a student failing for having too many
Plagiarism is a very serious academic violation, and your professor is likely to take a very strong stance against it. I certainly do! You might fail the assignment, fail the course, be reported to your dean, and even possibly suspended or expelled. Just don’t do it, under any circumstances!
That said, in the internet age, it can be pretty difficult to tell good sources of information from bad. My alma mater, Cornell University, runs an excellent website on Digital Literacy to help students learn how to navigate these matters. It is well worth a good look.
For a somewhat longer, but more entertaining, rant on the overuse and inaccurate use of the term, among other things, see Sense and Nonsense about Plagiarism by Steven Dutch.