To begin Understanding Media, McLuhan introduces 'hot and cool media,' explaining that one medium interprets another medium in the content. This means that the content of media is the expression of another form of media.
He then turns to analyzing the forms of common media from the 1960's. Among his discussion are the following media: Spoken Word, Written Word, Roads and Paper Routes, Comics, Advertisements, Games, Television, and even Weapons, among many more. Essentially, the discussion is an explanation of the manner in which different media express content through form—they all have this in common.
He argues that each of these media have their own inherent meaning, expressed through form. For instance, he argues that the secret meaning of film is that a movie should express the mechanics of human life in a highly rapid manner. The message of print is authority, especially as he discusses it politically, showing the development of modern attitudes toward print through a historical analysis all the back to the ancient oral traditions of law.
Famously, McLuhan observes that media is expressing its own meaning, and that content rarely has any real effect on people, whereas the medium has very strong effect. This is the book in which McLuhan says the well-known line, "The medium is the message."
