http://turnitin.com/research_site/e_faqs.html is a WWW site on plagiarism from Turnitin.com, a company which offers an expensive plagiarism detection service. I'm drawing attention to it because they get a fundamental fact wrong (see bold font):
What is plagiarism?
Simply put, plagiarism is the use of another's original words or ideas as though they were your own. Any time you borrow from an original source and do not give proper credit, you have committed plagiarism and violated U.S. copyright laws. (See our What is Plagiarism? page for more detailed information on plagiarism.)
What are copyright laws?
Copyright laws exist to protect our intellectual property. They make it illegal to reproduce someone else's expression of ideas or information without permission. This can include music, images, written words, video, and a variety of other media.
At one time, a work was only protected by copyright if it included a copyright trademark (the © symbol). According to laws established in 1989, however, works are now copyright protected with or without the inclusion of this symbol.
Anyone who reproduces copyrighted material improperly can be prosecuted in a court of law. It does not matter if the form or content of the original has been altered -- as long as any material can be shown to be substantially similar to the original, it may be considered a violation of the Copyright Act.
For information on how long a copyright lasts, see the section below on the public domain.
The assertion in bold is simply not true. Plagiarism is separate from and different than copyright. The implication that if you plagiarize you can be prosecuted in a court of law is a stretch. It's that kind of over statement that needs guarding against. For a more balanced and reasoned distinction between copyright and plagiarism, see "Is it Plagiarism or Copyright Violation" by Susan Dunn at http://www.webpronews.com/wpn-6-20030910IsItPlagiarismorCopyrightViolation.html