If you think communication as something that people do through messages, production of messages, sharing of messages and interpretation of them, then mass communication becomes the mass production of messages. In fact in some ways, book was the first mass-produced product when you can produce infinite number of copies of the exact same thing. Suddenly you can create a message and reach vast numbers of people by repeating the message by mass-producing the message to the huge number of diverse hetereogenous crowds. Different type of technologies have been very good at reaching very specific groups . Some of them reaching enormously varied and diverse groups. When you can mass-produce messages you can reach unbelievable number of people. If u think about what it would like to suddenly reach 2 hundred million people, remember that people watch big events like super bowl, landing of moon, inauguration. This is in human history , it is incomprehensible to think about what it would have been like to reach that many people once we take it completely granted. Sending a message between one city and another, could take across the ocean, but with mass communication you can send message from one person or one group to an infinite number of groups, countries all aroudn the world, instantaneously reaching large groups of people.
Type of mass communication varies primarily by what sort of technology we are talking about. Some mass communication will bypass time as an obstacle and some will bypass space. İf you think of a book or a magazine, the messages written down will be preserved, it will stay there so you can communicate with large numbers of people over a long period of time. Other types of tech will span vast amounts of distance whether we are talking about a radio broadcast, internet chat or satellite boradcast, can no longer have distance as an obstacle. Some type of mass communication moving to a very specific, specialied groups, say you want ot reach everybody who...