Avoid Plagiarism and Don't Take Someone Else's Ideas as Your Own
Plagiarism, in simple terms, is an attempt to use another person's ideas, words, or research as his own. This can be done in the form of an academic paper, a public speech, research task, or an advertisement for a product. There are many students who intentionally plagiarize and will copy whole sections of documents into their papers before attempting to submit them as their own, but many of them are unaware they also are committing plagiarism. This may cause just as many problems as those who are doing it purposefully.

The biggest rule in writing a paper is: when in doubt, always cite your source. You can never have too many citations. Of course, you want to use your own ideas and rationalize the material you are reading, but it is better to over cite than under cite. Footnotes and endnotes are good ways to make sure the words are being properly used as they should.
Many colleges and universities use the in-text citation system when there is going to be a bibliography or reference/works cited page at the end of the paper. This acknowledges that the writer is giving full credit to the writer who originally wrote the information and also allows the reader to have the full library source at the end of the article if this is needed.
Times when a person does not have to cite are when the ideas are coming straight from the author's head. This will be the case if an author is analyzing a situation or doing creative writing on a subject and explaining his understanding of the subject to the reader. Most literary works will contain many citations and several quotes which have come directly from the book. Many other works, however, will contain only in-text citations which have been properly referenced in the back of the article.
No matter how one chooses, the important thing to remember is never to take credit for work one has not written himself. If the writer did not have the words come from his own brain, he is basically stealing someone's product and this is theft, just as if it were any other product.