...it is inappropriate to cite Wikipedia as a source, because it is updated in real time. So from the time you cite a Wikipedia page to the time another person looks up your citation, the page may have changed. Citations are usually only applicable to static publications that do not change once they are published. Wikipedia is not a static publication, it is dynamic!—it changes every minute. In this sense, Wikipedia is not a reliable citable source.
How reliable is Wikipedia as a general resource? Very reliable. Most of the content on Wikipedia is accurate, and many subjects are updated faster than the news. But, Wikipedia is subject to being edited by almost anyone and any time, and the edits may be biased, out of date, incorrect, or malicious (vandalism). Therefore, it is advisable to double-check what you learn in Wikipedia against other sources.
Keep in mind that many volunteers monitor Wikipedia for errors. Since there are so many people reading the articles and monitoring contributions using the Recent Changes page, and using watchlists, incorrect information is usually spotted and corrected quickly.
Some argue that new errors are also introduced all the time, so that the overall accuracy rate is not really improving. Nevertheless, by encouraging people to help with correcting articles, validating content, and providing useful references, errors will eventually be greatly reduced.
The more accurate Wikipedia becomes, the more it attracts additional contributors. It is a self-reinforcing cycle!
As a new editor you may be wondering, "How can I help?" and be overwhelmed by the extent and scope of the entire Wikipedia project.
An introduction for new contributors is available at Help:Getting started which provides a self-guided road map of tutorials, introductions, training topics and other resources allowing you to learn more about how you can help improve the Wikipedia project.