http://www.copyright.gov/circs/circ01.pdf
I prefer sources that are short and to the point, with links allowing me to explore various topics if I need to. This piece goes over all of the basics of creating and maintaining a copyright license. While that is not the objective, typically, of someone taking a Creative Commons course, it helps to see this information from a pro-copyright perspective to understand all sides of the issue.
It's also a primary source, meaning that the department issuing the copyrights in the United States also wrote this piece, which means it should be as accurate as possible.