The short answer is that plagiarism is stealing. It may seem like a victimless crime, the writer really isn't taking a physical object, but he is claiming someone else's ideas as his own. Whether it's copying word for word or paraphrasing an original idea without citing the original author, plagiarism is using somebody else's work for the writer's personal gain. Students do this to try to get better grades, while professional writers might be tempted by plagiarism to make a quick buck.
Pressed for time, some writers will look for a shortcut to produce before a deadline. Students might think they are incapable of earning a good grade with their own writing and will copy someone else's. Often a writer will justify her use of plagiarism by claiming that she couldn't have worded an idea better herself. With so much content available on the Internet, the temptation is great to simply copy a paragraph or two into a word document, change the font style and size to make the writer's work appear uniform.
Teachers and editors are quite capable of recognizing a writer's work. A quick tip off is re-reading the same material over and over again from different writers. Different font styles and sizes are other blatant signs of plagiarism, as are hyperlinks left in a printed document. Once an editor or teacher suspects a work is plagiarized, simply typing a key word or sentence into a search engine will provide the original work. The more detailed the Internet search, the faster a teacher or editor will discover the plagiarized work. If time is of the essence, teachers and editors can use one of many plagiarism detection websites on the Internet.
Research is a necessary part of both academic and commercial writing, and it can seem impossible not to fall into the plagiarism trap. Writing out your research notes by hand, paraphrasing in point form, is the first step in converting someone else's idea into your own. Know your research inside and out before starting to write; then you can put your notes aside and write from your own thoughts instead of somebody else's. Finally, give credit where credit is due; make sure you write down all the source's details when you are consulting it. When presenting factual information or strong conclusions, cite your sources either in the text or as a bibliography.
Taking the easy way out and plagiarizing work has consequences beyond the work the writer is passing off as his own. Getting caught plagiarizing will affect a student's grade, as assignments will not be accepted; the student can even be expelled from his institution. Not having done the proper research will also mean the student is ill prepared when it comes to test and exam time. For professional writers the consequences can be worse. Whether it's loss of income or the tarnishing of a reputation, plagiarism can affect a writer's career for the worse.