A definition: "Plagiarism is using another person's words or ideas without giving credit to the other person. When you use someone else's words, you must put quotation marks around them and give the writer or speaker credit by revealing the source in a citation. Even if you revise or paraphrase the words of someone else or just use their ideas, you still must give the author credit in a note. Not giving due credit to the creator of an idea or writing is very much like lying."
Source: VirtualSalt http://www.virtualsalt.com/antiplag.htm
A humorous look at how to write in your own voice, how to paraphrase, when to cite; from the Foster Business School Writing Center at the University of Washington: