I started exploring these for my personal continuing education. I hope to get my Master's in something after homeschooling. I will probably have to do much of the work online. MOOCs are a way to dip your toe into online education without shelling out hundreds of dollars in tuition. I started out easy with something I already knew, physics.
I enrolled in How Things Work I from the University of Virginia offered through Coursera. The class "met" for 6 weeks. What happens is that you watch a series of video lectures. Some courses have reading assignments. This one did not. Then you complete the assignments. In my case it was a 10 question quiz each week. Boy were those quizzes hard. Anyone who says that you don't need grammar for science is mistaken. I diagrammed some of the questions to figure out exactly what was happening.
After the course is over, you get a certificate of completion if you meet the minimum requirements. Does it mean anything? Probably not to anyone except me. There are a few courses that a few colleges will accept for credit, but those are not free.
Pros: free, lots of choices, easy unenroll if the course doesn't suit you
Cons: credit not recognized, forums can be nasty places with mean people, very little or no instructor interaction