First of all I get a kick out of the name. Very clever. I think they morphed "academy" into a more individualized version of the word that does a great job implying a more personal version of a class. Great name. At least to me anyway. Udemy offers instructional videos, usually several in a series, that teach a certain subject. The subject matter expert develops the course and uploads it to Udemy. People interested in the topic, then pay a fee and watch the video series. One more step away from physical media like DVDs.
Paper books going to ebooks is quite similar to this leap from CDs and DVDs to online media watching. One less machine needed. One more password to remember. On the upside, watching the content can happen just about anywhere you can get an internet connection now.