What is a MOOC? This video from Dave Cormier will explain:
MOOC: Every Letter is Negotiable
creative commons licensed (BY) flickr photo by mathplourde: http://flickr.com/photos/mathplourde/8620174342
Because they are (mostly) free, online, and although delivered in real time when you "go to class" is up to you, MOOCs are an ideal way to investigate something you need to know more about, or are just curious about! After you have enrolled, how much work you do is up to you - no one is looking over your shoulder. If you want to watch every video, and read every file, you may, but you don't have to. If you want to work for a certificate, you may, but you don't have to. If you want to participate in the forums, you may, but you don't have to. This is self-directed learning!
This video from the New York Times is more than a year old, but is still an accurate description of the evolution of MOOCs.
I strongly urge you to consult the tag cloud at MOOC-List (and follow them on Twitter) or check the OpenCulture list of free online courses, and select a course that interests you. Perhaps your school doesn't offer this material? or not in such detail? or you're just curious about the field, and want to know more? A MOOC is a great way explore at your own pace, to dig deeper into a subject you already love, or just get a preview of a new world.