Wikipedia is useful for quick, low-stakes information, and the sources cited in its articles are potentially credible sources for research. The same cannot be said, however, of Wikipedia articles themselves.
From the Harvard Guide to Using Sources:
[W]hen you're doing academic research, you should be extremely cautious about using Wikipedia. As its own disclaimer states, information on Wikipedia is contributed by anyone who wants to post material, and the expertise of the posters is not taken into consideration. Users may be reading information that is outdated or that has been posted by someone who is not an expert in the field or by someone who wishes to provide misinformation. (Case in point: A few years ago, an Expos student who was writing a paper about the limitations of Wikipedia posted a fictional entry for himself, stating that he was the mayor of a small town in China. Four years later, if you type in his name, or if you do a subject search on Wikipedia for mayors of towns in China, you will still find this fictional entry.) Some information on Wikipedia may well be accurate, but because experts do not review the site's entries, there is a considerable risk in relying on this source for your essays.