The English word "plagiarism" comes from the French, meaning to steal or take hostage. Plagiarism is the act of using someone else's ideas or words without proper attribution, that is without giving credit it where it is due. When someone commits plagiarism, whether purposefully or accidentally, they are therefore endangering their authority and credibility as a writer because they are considered to potentially be a word or idea thief.
Examples of documents that take credit for someone else's work in this way are, unfortunately, numerous. Wikipedia, for instance, is what is known as a "socially constructed" website. Although its editors work very hard to ensure each page entry there is properly attributed and cited (tied back through the bibliography listing) to its original source, there are numerous pages where citations are incomplete or missing all together. This lack of consistent credibility in citation and attribution has forced most academic institutions to disallow Wikipedia entries as academically appropriate research sources.
Likwise, in his now famous New Yorker article "Something Borrowed," Malcolm Gladwell discusses the thing line between plagiarism and art, or the taking of something old to make something new. In this article, Gladlwell shares the impact of this on a British dramatist, who did not consistently offer attribution in regard to parts of her award-winning play "Frozen," which were taken both from the real life of psychiatrist Dorothy Lewis and news articles and videos written about her life. Although "Frozen" won a Tony award for the best play of 2004, the allegations of plagiarism against its author Bryony Lavery have caused her award-winning writing to become cloaked in controversy and discussion and her own professional success, which seemed to be on the rise, to diminish considerably.
The thing a writer must remember, whether they are a student in an English course, a social writer on Wikipedia, or an award-winning playwright, is that plagiarism is more using a similar idea or pattern of language. It is fraud. Because a plagiarist commits the act purposefully with the intention to deceive. Even claims of plagiarism can damage a writer's credibility and thus their academic or professional reputation.