TurnItIn.com has created an interesting and informative illustration of different types of plagiarism. It places plagiarism into 10 categories ranging from outright "cloning" to less obvious errors, with clear indicators of the frequency and seriousness of each type of offense.
This is an excellent resource to share with students -
http://turnitin.com/assets/en_us/media/plagiarism_spectrum.php
CTAAR has collected other resources, articles, and tools for detecting and combating plagiarism at our Plagiarism and Cheating workshop resources page.
