CS50 is Harvard University's introduction to the intellectual enterprises of computer science and the arts of programming, which is to say our introductory course in computer science for majors and non-majors alike here on campus, as well as down the road at Yale University as well. CS50X is simply an online adaptation of that same course that curriculatively, technologically is exactly the same. So if you're here in Cambridge or down the road in New Haven taking CS50 on campus, you're indeed taking CS50. But if you're anywhere else in the world online, you're taking CS50X as well. As for the CS50 marathon, this was simply a replay of all of CS50's most recent lectures back to back for anyone online who might have missed them and wanted to catch up at any point therein.