Plagiarism can sometimes be hard to recognize. Take a look at these sites to ensure that the papers and projects you take part in are not inadvertently plagiarized.
The Plagiarism Education Site - http://www.plagiarism.org/
Online Writing Lab at Purdue University - http://owl.english.purdue.edu/owl/resource/589/01/
For more information, check out the MLA, Turabian, Chicago or other style manual of your choice.