To discuss professionalism and writing, we should begin with the most obvious problem: plagiarism. Outright plagiarism is always a serious offense, and, when professors forgive it, it is usually because they see it as a terribly dumb and desperate act. In a composition class that I taught, I once had a foolish student plagiarize from someone in the same section (then he was surprised that I noticed—duh), and in the hallways I have heard students talk about "getting away with" blatant plagiarism on papers. Further, there are ample websites (I will not supply their URLs here, natch) where lazy students can download ready-made papers and turn them in for their classes. (Savvy professors can track down these papers readily, of course, perhaps by submitting a suspicious paper to http://turnitin.com/, a website devoted to helping teachers check submitted papers for originality.)
However, as a writing tutor for 20 years I discussed this issue almost daily with students, and I am convinced that many students plagiarize "accidentally"—that is, they fail to cite information they took from a source because they quickly, if tentatively, assess that the information they chose resides in a "gray area," and thus it might not need to be cited. They reason, "Why bother if I’m unsure?", or "Why risk doing it poorly?" Further, students frequently oversimplify this issue by rationalizing that the information appeared in an encyclopedia or in several books and therefore it need not be cited. In one case, I had a student say to me, "If it’s on the internet, by definition, it’s common knowledge, and therefore I don’t have to cite it." Such thinking causes me pain.
What such writers must realize is that one’s use and citation of sources has to be both reflective and discerning. The quality and context of your sources matter just as much as their content, and you are obliged always to assess that quality and consider whether and how best to establish that context. Your readers—especially your professors—will naturally be assessing quality and context in the act of reading, and they will expect you to have done the same. When using web-based sources, the responsibility to cite your material conscientiously remains, and in fact your responsibility to assess the credibility of the information increases simply because the material did in fact appear on the web. Further, if you see in-text citation of sources as a final, merely trivial step to writing rather than as an integral part, you are bound to slip up somewhere in your citation practice or lose track of the relationship between your own ideas and those of your sources. I am always surprised at the number of students who sit down with me to review a "complete" paper draft, yet with no sources cited. "Oh, I do that last," they say, then during our tutorials we invariably encounter problems that can only be reconciled by a better handling of source material. Stated simply, using sources well in your paper is not a matter of mere mechanics; it is the art of blending source material within the context of a focused argument as you write.
Unfortunately, the norm for many students is that they spend hours unreflectively surfing the web with an overly broad research focus, or they quickly Xerox anything that looks relevant in the library, then, 24 or 12 hours before the deadline, they sit down and start tapping madly into the word processor, sometimes simply lifting whole paragraphs from their sources and hoping that it looks like their own work, loudly assuring themselves and their friends that they "work best under pressure." If this is your technique, you will find that it fails you miserably when it comes to writing a thesis or working on a lengthy writing project on the job. Writing a long research paper in a day is a bit like pulling an all-nighter on Christmas Eve to crochet a quilt—the end product looks hurried and flimsy, and you can be sure that you have left many loose ends and produced a lousy Christmas gift.
An excellent guide, along with examples, about different types of plagiarism and how to avoid them resides here: