The evolving form knits together education, entertainment (think gaming) and social networking. Unlike its antecedent, open courseware — usually written materials or videotapes of lectures that make you feel as if you’re spying on a class from the back of the room — the MOOC is a full course made with you in mind.

The medium is still the lecture. Thanks to Khan Academy’s free archive of snappy instructional videos, MOOC makers have gotten the memo on the benefit of brevity: 8 to 12 minutes is typical. Then — this is key — videos pause perhaps twice for a quiz to make sure you understand the material or, in computer programming, to let you write code. Feedback is electronic. Teaching assistants may monitor discussion boards. There may be homework and a final exam.

The MOOC certainly presents challenges. Can learning be scaled up this much? Grading is imperfect, especially for nontechnical subjects. Cheating is a reality. “We found groups of 20 people in a course submitting identical homework,” says David Patterson, a professor at the University of California, Berkeley, who teaches software engineering, in a tone of disbelief at such blatant copying; Udacity and edX now offer proctored exams.

Some students are also ill prepared for the university-level work. And few stick with it. “Signing up for a class is a lightweight process,” says Dr. Ng. It might take just five minutes, assuming you spend two devising a stylish user name. Only 46,000 attempted the first assignment in Dr. Ng’s course on machine learning last fall. In the end, he says, 13,000 completed the class and earned a certificate — from him, not Stanford.

That’s still a lot of students. The shimmery hope is that free courses can bring the best education in the world to the most remote corners of the planet, help people in their careers, and expand intellectual and personal networks. Three-quarters of those who took Dr. Patterson’s “Software as a Service” last winter on Coursera (it’s now on edX) were from outside the United States, though the opposite was true of a course on circuits and electronics piloted last spring by Dr. Agarwal. But both attracted highly educated students and both reported that over 70 percent had degrees (more than a third had graduate degrees). And in a vote of confidence in the form, students in both overwhelmingly endorsed the quality of the course: 63 percent who completed Dr. Agarwal’s course as well as a similar one on campus found the MOOC better; 36 percent found it comparable; 1 percent, worse.

Ray Schroeder, director of the Center for Online Learning, Research and Service at the University of Illinois, Springfield, says three things matter most in online learning: quality of material covered, engagement of the teacher and interaction among students. The first doesn’t seem to be an issue — most professors come from elite campuses, and so far most MOOCs are in technical subjects like computer science and math, with straightforward content. But providing instructor connection and feedback, including student interactions, is trickier.

“What’s frustrating in a MOOC is the instructor is not as available because there are tens of thousands of others in the class,” Dr. Schroeder says. How do you make the massive feel intimate?