Oxford Dictionary defines a MOOC as "A course of study made available over the Internet without charge to a very large number of people."
One of the earliest "massively open online courses" was offered in 2008, by Canadian professors George Siemens and Stephen Downes. In 2012, the idea seized the public imagination when then-Stanford University professors Sebastian Thrun, Andrew Ng, Daphne Koller and others undertook an experiment to offer online lectures available to the public for free.
Since then, Thrun has left Stanford to start an online learning program called Udacity; Ng and Koller started Coursera. Professors at Harvard and MIT joined forces to start a program called edX.
A comprehensive list of all the buzz around MOOCs can be found here.
George Siemens reflected on MOOCs and the hype around them here.