This document defines and describes MOOCs (Massive Open Online Courses). It notes that MOOCs are online courses that are open to unlimited participation via the web, and include video lectures, readings, forums for interaction. The document outlines the history of distance learning and emergence of MOOCs in the 2000s. It distinguishes between xMOOCs and cMOOCs and their approaches. It then focuses on the MOOC platform edX, founded by MIT and Harvard, and how it hosts free university courses worldwide. It also describes open edX which allows other institutions to host their own courses using the edX open source software.