In the 21st century, the information and communication technology explosion increases the uses of digital devices for many purposes in the world of work and in formal and non-formal education. This study analyzes existing literature on the basis of the definition of the concepts, terminology used, differences, fundamental perspectives, benefits, disadvantages, and finally the similarities and differences of the e-learning (electronic learning), m-learning (mobile learning), and d-learning (digital learning). It reveals that e-learning and m-learning are subsets of d-learning. On the other hand, some learning tools could be considered as m-learning as well as e-learning.