MOOCs (Massive Open Online Course) are interactive online courses that typically are free and open to anyone with an Internet connection. Like earlier free online course offerings, e.g., MIT's OCW (Open CourseWare) initiative, they provide resources such as videos, recommended readings, and problem sets. They differ from these earlier online courses in two major ways: the courses are designed for online use instead of being copies of on-site existing courses, and they are structured around interactive social networks, called user forums.
Currently there are three major MOOC vendors: Coursera (www.coursera.org), edX (www.edx.org), and Udacity (www.udacity.com). While the format for the three is similar, Udacity differs from Coursera and edX in that it does not have a calendar-based schedule; students may start a course at any time.The figure below from Stanford (http://www.stanforddaily.com/2013/02/05/a-look-at-online-education-coursera-edx-and-udacity/online-education-page-1/) summarizes key components: history, number of universities, number of courses, number of students, and whether they are for-profit or non-profit.