Who hasn't heard of Wikipedia? A great information resource with lots of links to other resources, "Wikipedia is written collaboratively by an international group of volunteers. Anyone with internet access can write and make changes to Wikipedia articles."
For this reason, take care, as with all information resources, to verify accuracy and credibility. Be particularly careful about using Wikipedia information when you see messages like "The neutrality of this article is disputed" near the top of the page.They indicate that this could be a biased or inaccurate article.
In general, Wikipedia should not appear as a reference in a graduate level paper or project. Double check with your instructor if you have questions.