Massive online open courses (MOOCs) are a recent development in the education space. These are courses with a single professor (assisted by a few graders) who posts video lectures, quizzes, and homeworks, along with a class comprising hundreds or perhaps even thousands of students. For many MOOCs, registering is free but access to course materials requires registration. The professors do not grade student work. Grading is typically done through peer evaluation and automated grading systems.
A large number of people who sign up for MOOCs do so without any intention of taking the course, but rather, to access the course materials for current or later use. Completion rates for MOOCs are about 10%, but completion rate for MOOCs relative to the set of people who turn in the first homework are around 40-50%.
Keep in mind that, unlike online high schools, MOOCs are not accredited courses, and they do not offer the equivalent of college credit. They do offer certificates of completion (in some cases, they also offer certificates of completion with distinction). It is possible that these may be used to convince potential employers of skills or interests, but we do not have any evidence that they have successfully been used as such. We do not have direct information regarding MOOC participation being used by students.
MOOCs can be thought of as a middle ground between watching video websites like Khan Academy or other online videos or practice systems, and online high schools. Since they're in between, they have some of the advantages of both, but they also share the disadvantages.