This document provides an overview of massive open online courses (MOOCs). It discusses the origins of MOOCs from early open courseware projects at MIT and the formation of edX, Coursera and Udacity. Key aspects of MOOCs are described such as their open enrollment, lack of fees/prerequisites and format of online video lectures. Both potential benefits and concerns about MOOCs are outlined. The document also briefly introduces OER Commons, an online library for open educational resources.