Note: I'll be following up with a special article concerning Wikipedia, which is a different matter.A plagiarized article is one that is literally copied from another source. A lot of articles on this wiki (but not all!) are copied from sites like Monster Archives and Wikipedia.
Plagiarism vs References
If you write a paper, you have to put a section at the end called a bibliography. This contains all the sources for your information. If you were writing about ancient history you might site Plutarch, or if evolution you might site Darwin. You also insert a reference when you quote someone. If you quote a book, you would insert a footnote in a format roughly similar to: "Book, publisher, edition, date published, page/chapter/line."
It's important not to confuse these two types of references. If you copied your entire paper from Plutarch, and inserted him in the bibliography, you would get a zero. A reference is a source. If it's a quote, it's enclose in quotation marks with a note afterwards or in the source. We can quote people here, but it's not very professional to quote Wikipedia. Inserting "Wikipedia" in the references section is saying "I rewrote a Wikipedia article in my own words." That's why we have template:Wikipedia.
Problems with Plagiarism
This doesn't really refer to Wikipedia, I'll bring it up in a later post. This is referring to work from a site like Monster Archives.
Writing is an art. If you steal online fanart, you'll likely get in trouble for it. The reasoning behind it is that you took the work, claimed it as your own, and did not give credit to the original writer. Referencing isn't enough. Most people don't give their work a creative commons license, although because this is a wiki about copyrighted content that's an unbalanced proposition.
Taking work like that is a form of stealing, which is rude and morally wrong. It's better to rewrite it in your own words. At the very least place a notice on the top of the page. Plagiarism is also bad for the reputation of a wiki. It's better to just rewrite it in your own words.