Below is a list of the various levels of research sources and how they correspond to the instructor's protocol (i.e., with impact on your GRADE).
When you turn an essay in, the first thing I do is turn to the works cited page. If I do not see at least two A and two B sources, I return the paper. You can replace B sources with A sources, but not vice versa. Once you have at least two A and two B sources, it is fine to add in C and D sources for flavor or critique. If there are any F sources listed, I return the paper without reading it. Students have one chance to turn in a revision; the second time around, they are stuck with the grade warranted according to the quality of their sources. Obviously, if there is no works cited, I return the paper.
If the works cited page is acceptable, then I check to see if the paper meets the minimum page requirement. If there are double returns between paragraphs, oversized fonts, or other shenanigans, the paper gets returned.
A level sources are peer-reviewed. This means that before being published, the essay passed through a review process overseen by other professionals within the same field, who have the proper background knowledge to judge the essay's accuracy. A general rule of thumb is that peer-reviewed articles are sure to be found in (a) electronic databases linked to a university library research page -- specifically MLA International Bibliography, J-Stor, and Project Muse. Also, these sources are authentic if they are (b) nonfiction scholarly books published by an academic press and housed in a university library. Examples are Nineteenth-Century Prose, dis(Closure), Literature Compass, Edgar Allan Poe Review.... or Shakespeare and the Invention of the Human (Bloom), The Colorful Conservative (Lopez), Anxiety of Influence (Bloom), The Western Canon.
B level sources are editorially reviewed. This means that the author had to submit work to an editor prior to getting it published. The editor may or may not have extensive background knowledge relevant to the article's content but nonetheless has experience judging quality prose. Usually B level sources are periodicals with a quick turn-around time because they are meeting the needs of a lay audience interested in current events. To be a B level source, the article must have an attributed author. The only other possibility is if an institutional authority published a report and the contributors are listed in it (ex., Cato Institute's report on the economic stimulus, United Nations' report on refugees.) Typical examples of B sources are New York Times, Washington Post, CNN, LA Times, Daily News, Guardian, Wall Street Journal, Atlantic Monthly, USA Today, FOX News, Al-Jazeera, Politico, Better Homes & Gardens, Publishers Weekly, Rolling Stone, Readers Digest, Psychology Today, Guns & Ammo, Field & Stream, Cosmopolitan, Playboy, Spin......
C level sources are nonfiction prose attributed to a specific author who has some credentials or expertise in the subject matter, but whose work has not been reviewed by someone else prior to publication. Examples of this would be professors' blogs, domains ending in "edu," self-published professional material, podcasts, Glenn Beck, Bill O'Reilly, Rachel Maddow, Ann Coulter, Rush Limbaugh, Jon Stewart, Arianna Huffington, Stephen Colbert, and Barbara Walters. It is a fine line but you should pay some attention to the amount of time this self-published author has spent in the field of commentary. Established commentators are generally going to be more relevant to your paper than people who have not had extensive experience being criticized.
D level sources are reference materials with no attributed author, but at least with some institutional imprimatur. (An imprimatur means license to be published.) Examples of this would be dictionaries, encyclopedias, thesaurus, almanacs, Bartleby, IMDB.com, Amazon synopses, online databases, Ask.com, About.com, or wiki discussion boards conducted among learned folk. I also include E-notes, Sparksnotes, Cliffnotes, Barron's notes, etc. If you use these you are usually wasting your time since such sources will rarely enhance your essay--and may, in fact, undermine your credibility if too many D references surface in succession.
F level sources are materials with such questionable authority that use of them in your paper could trigger a low grade, even an F. The primary example is Wikipedia. You should feel free to use it as a starting point for your research but you should always follow it with subsequent referrals to more authoritative work. You do not need to list Wikipedia if you only used it as a starting reference point. It is probably better not to cite it or list it anywhere in your paper. NEVER quote or scan text from such sites into your paper, as this is a clear violation of academic honesty, even if you want to cite it, for the simple reason that academia views such sites as illegitimate. Other examples of F sources would be AOL Hometown, message boards, discussion sites, promotional emails, chain letters, or listservs.