The PC Student Handbook defines plagiarism as:
The use of paraphrase or direct quotation of the published or unpublished work of another person without full and clear acknowledgment.
Properly documenting research sources is important because it ensures that:
-Credit for information originally provided elsewhere is given
-Others can find the same information again
-Plagiarism does not take place
Writing A Great Research Paper
Student's Guide to Writing College Papers by 
The Complete Guide to Referencing and Avoiding Plagarism by 
The Wadsworth Guide to Research