Online course work has been a staple of American higher education for at least a decade. But over the last few years, a new, more ambitious variant known as a MOOC -- massive open online course -- has challenged traditional assumptions of what an online course can be. MOOCs have exploded in that short time, redefining who can enroll in college courses, as well as where, when and even why people take online classes.
Available globally to hundreds of thousands of people at a time, these classes depend on highly sophisticated digital technology, yet they could not be simpler to use. Signing up takes less time than creating an iTunes account. You can create a user name and password and start exploring the rapidly expanding course offerings.
The major Web sites already provide dozens of courses, as diverse as basic calculus and European intellectual history. It is both new and experimental, and as much as MOOCs have evolved since beginning in recent years, enthusiasts expect many more changes. From an early focus on technical and scientific courses, for instance, offerings now include the humanities and social sciences.
While there are some significant differences among the major MOOC Web sites, they share several main elements. Courses are available to anyone with access to the Internet. They are free, and students receive a certificate of completion at the end. With rare exceptions, you cannot earn college credit for taking one of these courses, at least for now.
"For a decade, people have been asking, 'How does the Internet change higher education,' " said Edward B. Rock, a law professor at the University of Pennsylvania who is the institution's senior adviser on open course initiatives. "This is the beginning. It opens up all sorts of possibilities."
Navigating the world of MOOCs begins with three major Web sites.
Harvard and the Massachusetts Institute of Technology created this nonprofit joint venture in May 2012. It has already offered dozens of courses in subjects as diverse as physics, computer science, engineering, literature, ethics, law, medicine and economics.
Twenty-nine universities have signed up to participate, including the University of California, Berkeley; the University of Texas, Austin; Georgetown; Cornell; the Berklee College of Music; the University of Toronto; and the University of Kyoto.
Courses are offered for a designated period of time, with lectures and reading assignments provided in weekly segments. Videos of lectures are generally augmented with exercises, quizzes, labs and simulators. Like other platforms, edX emphasizes interactivity.
You can audit a course -- meaning you don't take exams or do writing assignments -- or you can fulfill all of the requirements to earn a certificate of completion.
Each course's home page provides an estimate of how many hours a week the course will require. Workloads vary widely. A Global History of Architecture, an M.I.T. class, requires at least five hours a week. Introduction to Computer Science, Harvard's traditional introductory course, asks online students to complete eight problem sets, each of which will take 15 to 20 hours, along with two quizzes and a final project.
Two computer science professors at Stanford began this commercial venture in April 2012. The original partners were Stanford, Princeton, the University of Pennsylvania and the University of Michigan. Seventeen months later, Coursera has partnerships with 84 universities and offers more than 400 courses.
Yale, Duke, Wisconsin and the University of Chicago are among the participants, as are the University of Edinburgh and the École Polytechnique in France.
Because courses are free, Coursera hopes to generate revenue in other ways, like linking corporations with students who have learned specific skills. Coursera does not formally offer the option of auditing a class, but people certainly can. Anyone can simply watch the videos and do some, all or none of the reading and homework; you just would not receive a certificate at the end.
Like edX, Coursera emphasizes interactivity, both online and by helping to organize in-person study groups in cities around the world. There are also online discussion groups, peer-to-peer forums and peer assessments of writing assignments in some classes.
Among the 70 or so courses that began this month were Introduction to International Criminal Law; Linear and Integer Programming; and Exploring Beethoven's Piano Sonatas. Courses run for a set length, with material provided each week. Some last four or five weeks, while many go for 12 to 15 weeks.
This is also a commercial start-up. It was begun by several people, including another Stanford professor, in January 2012. Udacity's offerings are more limited, with a strong emphasis on science, math and computer science. There is also a sprinkling of business, psychology and design courses. About 30 courses are available.
The courses do not start or end on specific dates and they do not follow a weekly pattern of lectures and assignments. They are entirely self-paced: You start whenever you like, and you work through the material as quickly or as slowly as you please.
On the home page of an introductory course in computer science, Professor David Evans of the University of Virginia encourages students to collaborate and explains how his MOOC class is different from a traditional course.
"We intersperse our video segments with interactive questions," Dr. Evans writes. "There are many reasons for including these questions: to get you thinking, to check your understanding, for fun, etc. But really, they are there to help you learn. They are NOT there to evaluate your intelligence, so try not to let them stress you out."
There are sites that offer things similar to massive open online courses. Udemy offers a huge variety of courses, primarily in practical subjects like Excel software or using an iPhone camera. Most of these courses carry fees, and most of the teachers are not university professors. The Kahn Academy Web site offers more than 4,000 short videos on a variety of academic subjects.
Al Filreis, a professor of English at the University of Pennsylvania whose MOOC on modern and contemporary American poetry attracted 21,000 participants last fall, offered some tips for choosing from the hundreds of courses available:
■ Watch the professor's introductory video.
■ Examine the syllabus to determine how much work will be required and the level at which the course will be taught.
■ Try to determine how involved the professor will be in the course.
■ Research the professor by looking at the traditional university courses he or she teaches and reading an article or essay by the professor.
■ Search the Web for course reviews. A few Yelp-like Web sites have been created to offer individuals' assessments of these courses, like CourseTalk.
Much has been made about the large dropout rate in this kind of class; as many as 90 percent of those who register for a class do not complete it. John C. Mitchell, Stanford's vice provost for online learning, said surveys indicated that about 25 percent of MOOC enrollees do much of the work, without necessarily finishing the course.
Many enthusiasts insist that there is nothing wrong with trying out several courses and completing only one, or simply watching lectures until you run out of time. "Sign up for anything that appeals to you and quit when you get bored," said Cathy N. Davidson, an English professor at Duke who teaches and takes open online courses.
Michael S. Roth, the president of Wesleyan University, is teaching a course in European intellectual history on Coursera. "People have to get over the idea of signing up for a course and not finishing it," he said. "We all have to get over feeling guilty. There is no right way or wrong way to use an online class. This is a great platform for lifelong learning."interact
This article originally appeared in The New York Times.