MOOC stands for "Massively Online Open Course". MOOCs are the Big New Thing in higher education these days ... and they're actually really damned cool.
These are online courses offered by professors at schools like MIT, Ohio State University, and other institutions big and small. Any student anywhere in the world (provided he or she has a sufficiently robust Internet connection, of course) can sign up for a MOOC for free. Tens of thousands of students might enroll in a single course, thus the "massive" part of the title.
Most MOOCs don't offer transferrable college credit (some do, and more will in the future because of demand) but the idea here is that you get to learn about something you're interested in from someone who is enthusiastic to teach it. As with any other endeavor, what you get out of a MOOC is almost entirely dependent on what you're willing to put into it.
For instance, did you avoid calculus in high school for fear of getting a bad grade? Have you regretted that decision because all your scientist friends now think you're a lightweight? Then take MOOCulus and earn your intellectual bragging rights. Mathematical!
But maybe calculus isn't your idea of a fun education experience. Maybe you're more interested in something creative, like digital storytelling? Then check out DS106, which has spilled out of its laboratory at the University of Mary Washington and thrives all across the Web.
There are several sites you can visit to discover, sign up for and access MOOCs. My favorite is Coursera at https://www.coursera.org. 62 universities collaborate with Coursera to offer over 300 courses; at present, over three million people are taking classes through their system on a wide array of topics, including engineering, art, literature, medicine, biology, social sciences, mathematics, business, and computer science. The courses themselves are free, but for the courses that offer credit or certificates, a fee must be paid to obtain that credential.
Another option is Udacity at https://www.udacity.com, which at present has a smaller pool of courses than Coursera. Udacity's MOOCs are free, but as with Coursera, students seeking formal college credit will need to pay for the credential.
A third good MOOC provider is edX at https://www.edx.org. edX was begun as a collaboration between MIT and Harvard and is a not-for-profit enterprise. The system now offers free courses from those universities plus Berkeley, the University of Texas, McGill, Australian National University, Wellesley, Georgetown University, the University of Toronto, EPFL, Delft University of Technology, and Rice University.
You can find other resources at the Massive Open Online Courses Guide and Affordable Online Learning.