A Massive Open Online Course (MOOC) is a model for delivering learning content online to any person who wants to take a course, with no limit on attendance. The MOOC industry is growing rapidly, fueled by students interested in free higher-education learning, and the universities and venture capitalists willing to fund the courses. This paper compares the MOOC format with the format of a Mainstream Online University Course (MOUC). Comparing MOOCs with MOUCs will help those readers familiar with online courses to understand the unique characteristics of MOOCs, and their implications for learning. MOOCs are compared to MOUCs on the following dimensions: Course enrollment; Openness; Price; Content media; Discussion forums; Assessment; and Instructor contact. MOOCs are already effective substitutes for learning that is not for academic credit. Several experiments are under way that may result in MOOCs contributing to for-credit courses or delivering them entirely.