Wikipedia has a wealth of information, and is probably one of the first places people go for information on a topic. While surfing Wikipedia is great to learn about something, it is not an authority, and it is not edited or peer reviewed! Before Wikipedia, teachers had a hard enough time convincing students to not cite things from the internet, because anyone can write it and publish it. In fact, several of my colleagues have to include a penalty on their paper writing assignments, because it is so common to see Wikipedia used as a Reference.
When you cite something in your paper, it means that the information or idea you are presenting is not your own, and that the reader can find more information about what you are talking about by looking up your reference. However, since anyone can edit Wikipedia, it is entirely possible that you created that post in Wikipedia to support your idea or topic. I realize most people wouldn't go to this much trouble, but it is possible. It is also possible that someone else has edited the article by the time your teacher checks your reference.
By sticking with books and journal articles, you know immediately who wrote it, and who published it. It is not possible for a published work to be changed or altered, even if it contains mistakes. If you cite an error in a book you have read, then that error will be there when your teacher takes a look. You can also have confidence in the fact that it was edited and reviewed by several people before being printed for a general audience.
However, Wikipedia is a better place to learn from than other internet sites because it often includes citations and references in its posts. If you are a student doing research, then it is these citations that you should be interested in. While it is too common for Wikipedia citations to include other websites (a huge no-no), it is also common that they will cite books, government documents, and the occasional academic journal. These last three types of sources are what should make up the bulk of your references section. Remember that most of your teachers attended school when the internet was new or non-existent and may hold on to these classic types of sources.
To sum it up:
- Never cite Wikipedia, use it as a source to find relevant references
- Only cite websites that are an authority, including .edu, .gov, or some other institutional sites
- You shouldn't cite this site (I know, its funny :) )
- Instead you should use this site for consulting, to learn something, and then cite an appropriate textbook, such as the one you are using for the class. Also check my references section to see which books I have used when making this site