Nick McKeown is teaching one of them, on computer networking, with Philip Levis (the one with a shock of magenta hair in the introductory video). McKeown sums up the energy of this grand experiment as he gushes: "We're both very excited." Casually draped over auditorium seats, the professors also acknowledge that they are not exactly sure how this MOOC stuff works.

"We are just going to see how this goes over the next few weeks," McKeown says.

Traditional online courses charge tuition, carry credit and limit enrolment to a few dozen to ensure interaction with instructors. The MOOC, on the other hand, is usually free, credit-less and, well, massive. Because anyone with an internet connection can enrol, faculty can't respond to students individually. So the course design - how material is presented and the interactivity - counts for a lot. As do fellow students. Classmates may lean on one another in study groups organised in their towns, in online forums or for grading work.

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 understand the benefit of brevity: eight 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 non-technical 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.

Ray Schroeder, director of the Centre 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 such as computer science and maths, with straightforward content. But providing instructor connection and feedback, including student interaction, 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," Schroeder says. How do you make the massive feel intimate?

That's what everyone is trying to figure out. Many places offer MOOCs, and more will. But Coursera, Udacity and edX are defining the form as they develop their brands.

Coursera casts itself as a "hub" for learning and networking. The learning comes gratis from an impressive roster of elites offering courses from computer science to philosophy to medicine. Not all are highbrow or technical; "Listening to World Music" from the University of Pennsylvania aims to broaden your iPod playlist.

While Coursera will make suggestions, Ng says, "ultimately all pedagogical decisions are made by the universities". Most are adapted from existing courses: a Princeton Coursera course is a Princeton course. But the vibe is decidedly Facebook - build a profile, upload your photo - with tools for students to plan "meet-ups" with Courserians in 1,400 cities worldwide. These gatherings may be bona fide study groups or social sessions.

Udacity has stuck close to its maths and computer science roots, and emphasises applied learning, such as "How to Build a Blog" or "Building a Web Browser". Job placement is part of the Udacity package. "The type of skills taught in computer science, even at elite universities, can be very theoretical," Stavens explains.

In a poke at its university-based competition, Stavens says they pick instructors not because of their academic research, as universities do, but because of how they teach. "We reject about 98 per cent of faculty who want to teach with us," he says. "Just because a person is the world's most famous economist doesn't mean they are the best person to teach the subject."

Stavens sees a day when MOOCs will disrupt how faculty are attracted, trained and paid, with the most popular "compensated like a TV actor or a movie actor". He adds that "students will want to learn from whoever is the best teacher".

That means you don't need a PhD. "Landmarks in Physics", a first-year college-level course, is taught by Andy Brown, a 2009 MIT graduate with a BS in physics. "We think the future of education is guys like Andy Brown who produce the most fun," Stavens says. Brown's course is an indie version of - filmed in Italy, the Netherlands and England, with opening credits for "director of photography" and "second camera and editor".

Whether explaining what the ancients believed about the shape of the earth or, in Thrun's statistics course, why you are unpopular, statistically speaking, voiceovers are as non-threatening as a grade school teacher.

"You feel like you are sitting next to someone and they are tutoring you," says Jacqueline Spiegel, a mother of three with a master's in computer science from Columbia who has enrolled in MOOCs from Udacity and Coursera. While taking "Artificial Intelligence", she discovered she liked tackling assignments in online study groups.

The class was tough and took "an embarrassing amount of time", says Spiegel, who found that consuming lectures by smartphone during her 14-year-old's 6am ice skating sessions worked less well than being parked at a desktop. Her effort was huge - some 22 hours a week - but rewarding. Spiegel befriended women in India and Pakistan through Facebook study groups and started an online group, CompScisters, for women taking science and technology MOOCs.

If Udacity favours stylish hands-on instruction, edX aims to be elite, smart and rigorous; don't expect a gloss of calculus if you need it but never took it. Only Berkeley and the University of Texas System have been admitted to the club.

Assignments that can't be scored by an automated grader are pushing MOOC providers to get creative. Coursera uses peer grading: five people grade your assignment; in turn, you grade five assignments.

But what if someone is a horrible grader? Coursera is developing software that will flag those who assign very inaccurate grades and give their assessment less weight. Mitchell Duneier, a Princeton professor, is conducting a study that compares peer grading of the final exam in his sociology MOOC on Coursera last summer with the grades he and his course assistants would have given the students.

So, what do you get for your effort? Do you earn a certificate? A job interview? Or just the happy feeling of learning something?

"If one is going for the knowledge, it's a boon," says Schroeder of the University of Illinois. "If one is looking for credit, that is one of the challenges. How do we fit this into the structure of higher education today?"