The medium is the message.
McLuhan famously argues in Understanding Media that the manner by which information is presented to an audience is the only important feature in determining the effect on the audience. Put more succinctly, McLuhan believes that it's the type of medium that carries the significance.
Media are everywhere.
The effect of a medium is an environment, like a force field one might say, so any transmission of data or energy that produces a force field of effect is a "medium" for McLuhan—even lightbulbs. Therefore, the human experience is highly influenced by media.
Content does not effect the general population.
McLuhan's most controversial idea was that content does not matter - that only media really affects people. Therefore, whether the general public watches gory, violent movies, it doesn't affect them. The only lasting effect of the movie is the expression of the form of the medium of film. In other words, all movies have the same effect on the public.
