MOOC stands for Massive Open Online Course. There are two main types of MOOCs - xMOOCs which are based on traditional university structures and cMOOCs which are based on connectivist learning models favoring collaboration. Some key advantages of MOOCs are that they have relaxed requirements, use video formats, provide accessibility to top university content, allow repetition, and are high quality, feasible, self-paced, and enable online collaboration. Popular MOOC platforms include Coursera, edX, Udacity, and FutureLearn which provide courses from universities worldwide.