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.