Wikipedia can be edited by visitors, including you and me, without registering (except for a few protected pages). Not all contributors are experts, thus they may introduce factual errors.
Due to the open nature of Wikipedia, sometimes there are a small number of vandalism that damages a page, but most of these are quickly reverted by other visitors and some wikiholics that dedicate themself to monitoring the changes page.
Basically, when using Wikipedia (or generally any encyclopedia), take it with a grain of salt. Not even paid encyclopedias, which is written by experts, are free of errors; and I think there were some research some time ago that found that the number of errors between Wikipedia and regular encyclopedia is not really differ that much. They concluded that the open nature of Wikipedia and the large number of visitors allowed errors to be spotted and corrected quickly.