Plagiarism is stealing the work of someone else and pretending like it is your own. All of the sources used must be cited. It also consists of using another persons idea and attempting to pass it off as an original idea. When using a quote or a portion of someone's work it must be put in quotation marks. Changing certain words but keeping the overall structure of a sentence the same, is still considered plagiarism. Even if credit is given, the bulk of the words can not be the same due to copyright law and fair use. See the links below for further information on plagiarism.