It isn't unusual for students to plagiarize unintentionally, simply because they don't know when or how to cite sources in a paper. Consult these sites to avoid plagiarism.

UGA's Academic Honesty Policy: Definition of Plagiarism
The OWL at Purdue: Is It Plagiarism Yet?
The OWL at Purdue: Quoting, Paraphrasing, and Summarizing

For more extensive information on avoiding plagiarism, see Plagiarism Resources at UGA