To avoid plagiarism, a general guideline is that you should write the assignment in your own words and when you use ideas from another source you need to cite your source.
The following definitions come from UNC Dean of Students:
Plagiarism is the act of attempting to pass another's words or ideas off as one's own. It is also using another's words or ideas and not giving the original author credit for them. Plagiarism ranges from using a single word, in the case of a word or phrase an author has "coined," to claiming ownership of an entire document written by someone else. It can also apply to works written by one author that have been substantially reworked or edited by another.
The university further defines plagiarism as "the act of appropriating the written, artistic, or musical composition of another, or portions thereof; or the ideas, language, or symbols of the same and passing them off as the product of one's own mind. Plagiarism includes not only the exact duplication of another's work, but also the lifting of a substantial or essential portion thereof." This definition and other important policy information is available by accessing the Student Code of Conduct.
Quoting and Paraphrasing
Quoting is using another's exact words either to add authority to the concept, theory, or information one is trying to portray, or because that author stated so clearly what one is wishing to add to one's document that one could not say better oneself.
All of the author's words must be enclosed in quotation marks and the author must be credited at the end of the quote.
Paraphrasing is utilizing an author's ideas in the forwarding of one's own ideas. When paraphrasing, the reader should be able to distinguish one voice from another. This is accomplished by offsetting the paraphrased material by enclosing it in quotation marks or changing the left and right indent spacing. One must still credit the original author at the conclusion of the borrowed text.
One also needs to be aware of the term "paraplagiarism," which was coined recently to describe the act of rearranging or mildly modifying an author's work in order to call it one's own.