The Oxford English Dictionary defines plagiarism as:
"...the wrongful appropriation or purloining, and publication as one's own, the ideas or the expression of the ideas (literary, artistic, musical, mechanical, etc.) of an other."
The university policy on plagiarism is covered in Appendix D of the Student Handbook, Section 1 - Academic Offences. Individual departments may also have their own policies dealing with this issue.
A good discussion on avoiding plagiarism can be found in the section on Researched Writing in The Bedford Handbook (Hacker) as well an in other style manuals in the library.
If you suspect that you may have accidentally plagiarized material TALK WITH YOUR PROFESSOR.
In addition to printed materials, many colleges and universities have created web sites to help students. A few sites for avoiding plagiarism are: