Yes, anyone can post on Wikipedia (assuming they have internet access and all the sort of stuff). Particularly disruptive people are sometimes banned, but it's pretty easy to get around. And although you can register a username on Wikipedia, it is not necessary and you can post anonymously if you wish. Because of all this, it's definitely possible the information you read there is not accurate. The same goes for the internet in general. That's why Wikipedia is generally not a good source for research papers. However, it's a great resource if you just want a quick overview of a topic from which you can explore more specific topics from other sources. I like to use it when I don't know what I don't know about a topic. In general, use common sense and consult other sources (especially offline sources with an author listed) if you want to improve your chances of getting accurate information. In Wikipedia's defense, no source of media is immune from including inaccurate information. One of the pluses of Wikipedia is that not only can anyone post, anyone can correct information as well. Check out their FAQ for further info: http://en.wikipedia.org/wiki/Wikipedia:FAQ

We are all encouraged to provide accurate, referenced information to the articles in Wikipedia. It is written by me and you, and everyone else who can provide evidence for the information we wish to post there. I am proud of the postings I have provided to help inform other people.