Free encyclopedia written by voluntary authors; quality of articles is always controlled, so there could not arise wrong articles, even if they are some manipulated, free authors controll and correct the changing (in few minutes) and you can be sure that articles are truthful. But be aware of the "articles history" there you can see changelog. To reduce more faults you also can compare to other encyclopedias. If you want to use Wikipedia for exams or skilled works you have to know that it is only secondary literature and you have to use others sources (primary sources) for better work. In general most Wikipedia articles are better than most non-free encyclopedia volumes. If you want know more about Wikipedia read also: http://en.wikipedia.org/wiki/Wikipedia
Wikipedia is a collaboratively edited, multilingual, free Internet encyclopedia that is supported by the non-profit Wikimedia Foundation. Anyone who can access the site can edit almost any of its articles.
As with any publicly edited and created content website you must use caution and verify anything you see as suspicious or untrue.