|This page in a nutshell: Do not use a Wikipedia article as a source for another Wikipedia article, even when describing Wikipedia.|
Wikipedia is not a reliable source. Wikipedia can be edited by anyone at any time. This means that any information it contains at any particular time could be vandalism, a work in progress, or just plain wrong. Biographies of living persons, subjects that happen to be in the news, and politically or culturally contentious topics are especially vulnerable to these issues. Edits on Wikipedia that are in error may eventually be fixed. However, because Wikipedia is a volunteer-run project, it cannot monitor every contribution all the time. There are many errors that remain unnoticed for days, weeks, months, or even years. Therefore, Wikipedia should not be considered a definitive source in and of itself.
The same applies to Wikipedia's sister projects, as well as websites that mirror or use it as a source themselves, and printed books or other material derived primarily or entirely from Wikipedia articles.
Articles are only as good as the editors who have been editing them—their interests, education and background—and the efforts they have put into a particular topic or article. Since we try to avoid original research, a particular article may only be as good as (a) the available and discovered reliable sources, and (b) the subject matter may allow. Since the vast majority of editors are anonymous, you have only their editing history and their user pages as benchmarks. Of course, Wikipedia makes no representation as to their truth. Further, Wikipedia is collaborative by nature, and individual articles may be the work of one or many contributors over varying periods. Articles vary in quality and content, widely and unevenly, and also depending on the quality of sources (and their writers, editors and publishers) that are referenced and/or linked. Circumstances may have changed since the edits were added.
It also helps to look at the article's editing history (it may have changed drastically over time; you can identify individual contributions and their contributors by user name), and the article's talk page (to see controversies and development).
To be sure, Wikipedia is a good springboard from which to launch your own research, but ... caveat lector.