A Surge in Growth for a New Kind of Online Course 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. Coursera 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.