Plagiarism and Copyright Violations

We all know students and writers should avoid plagiarism and violating someone else's copyright, but even so, it happens all the time. Were I a teacher, I would find plagiarism to be the more serious problem. If students plagiarize, they are using someone else's words, not doing their own research, and therefore, not learning. If a plagiarized paper slips past me, I may give an undeserving student an A, which could skew the curve, with the result that the person who turned in the next best paper, one done without plagiarism, would suffer in his grade. So plagiarism is unfair to other students. As a writer, someone violating my copyright means I don't get paid for my original work. On the Internet, it also means that any correction I make to the material is not included in the illegal copy.

There are more profound and moral reasons why copyright violations and plagiarism are wrong, but these are what I see as the practical ones.

Unfortunately, it is sometimes easy to plagiarize even without meaning to. Phrases cling to my memory and I can't remember where I heard them. Was it a dream? Was it something I said? Or was it something I read somewhere I can't remember? If I write it and it's the last, I have inadvertently plagiarized. Copyright violations are a bit harder to commit unwittingly and are illegal. However, if I see something that says it's in the public domain, chances are I'll believe it and assume using it doesn't violate copyright. It's also difficult to distinguish the point at which copying (with credit to the author) stops being "fair use" and becomes a copyright violation. But it seems ironic that academic institutions with warnings against (the sometimes difficult to avoid) plagiarism will occasionally use material that's clearly under copyright without express permission from the copyright holder.
Note: fair use does not mean that if you don't make money from it, you may use it.

Copyright
Copyright in the U.S. - A U.S. Library of Congress article with FAQ and more about copyright, including the Copyright Law in the U.S.
Ancient / Classical History Forum Discussion of Copyright

Plagiarism About.com's Women's History Guide has a fine selection of plagiarism resources, including ones on enabling teachers to spot plagiarism.