A MOOC is a Massive Open Online Course, the ambiguity of each of these letters mean the term is contested, however we can pick out some common patterns.
Massive implies education at scale, and much of the political and corporate heat and light have been around the scale. Open exists on a continuum, from open to anyone and free at the point of use to openly licensed CC content. While the degree to which it is open is one of the most contested areas within the MOOC world, Online is probably less contested as it is a vital enabler for scaling. However, what is put online is contested with the offer ranging from video capture of face-to-face lectures to integrated Virtual Learning Environments. This flows into one of the most contested elements – Course. On one level Course simply indicates a structure, a start and end point. What is contested is the underlying pedagogical model.
Approaches are not always binary, but fall into two general camps, those emphasising the co-creation in supportive peer communities building on the affordances of social media (e.g Twitter, aggregated blogs feeds) (cMOOCs), and those structured learning events (often video) and viewed as an extension of the lecture experience (xMOOCs). While these are most prominent in the literature, other formulations exist, for example vMOOCs emphasise vocational study, while others have dropped all but the online O and the C, for example SPOCs are small private online classes. While this domain is likely to remain contested and new acronyms flourish, the common aspects remain the same, the potential to scale a structured offer enabled by free or open online content.
See the contested WikiPedia Page on MOOCs at http://en.wikipedia.org/wiki/Massive_open_online_course
Here is a useful table from OpenUpEd that provides more detail http://www.openuped.eu/images/docs/Definition_Massive_Open_Online_Courses.pdf