McLuhan uses interchangeably the words medium, media and technology. For McLuhan a medium is "any extension of ourselves", or more broadly, "any new technology". In addition to forms such as newspapers, television and radio, McLuhan includes the light bulb, cars, speech and language in his definition of "media": all of these, as technologies, mediate our communication; their forms or structures affect how we perceive and understand the world around us.
McLuhan says that the conventional pronouncements fail in studying media because they pay attention to and focus on the content, which blinds them to see its actual character, the psychic and social effects. Significantly, the electric light is usually not even regarded as a media because it has no content. Instead, McLuhan observes that any medium "amplifies or accelerates existing processes", introduces a "change of scale or pace or shape or pattern into human association, affairs, and action", resulting in "psychic, and social consequences"; this is the real "meaning or message" brought by a medium, a social and psychic message, and it depends solely on the medium itself, regardless of the 'content' emitted by it. This is basically the meaning of "the medium is the message".
McLuhan, to show the flaws of the common belief that the message resides or depends on how the medium is used (the "content" output), uses the example of mechanization (machinery to assist the work of human operators), pointing out that regardless of the product (i.e. cornflakes or Cadillacs), the impact on workers and society is the same. In a further exemplification of the common unawareness of the real meaning of media, McLuhan says that people "describe the scratch but not the itch." As an example of so called "media experts" which follows this fundamentally flawed approach, McLuhan quotes a statement from "General" David Sarnoff (head of RCA), calling it the "the voice of the current somnambulism". Each media "adds itself on to what we already are", realizing "amputations and extensions" to our senses and bodies, shaping them in a new technical form. As appealing as this remaking of ourselves may seem, it really puts us in a "narcissistic hypnosis" that prevents us from seeing the real nature of the media.
McLuhan also says that a characteristic of every medium is that its content is always another medium.
The impact of each medium is somewhat limited to the previous social condition, since it just adds itself to the existing, amplifying existing processes. Therefore different societies may be differently transformed by the same media
The only possible way to discern the real "principles and lines of force" of a media (or structure), is to stand aside from it and be detached from it. This is necessary to avoid the powerful ability of any medium to put the unwary into a "subliminal state of Narcissus trance," imposing "its own assumptions, bias, and values" on him. Instead, while in a detached position, one can predict and control the effects of the medium. This is so difficult because "the spell can occur immediately upon contact, as in the first bars of a melody". One historical example of such detachment is Alexis de Tocqueville and the medium of typography. He was in such position because he was highly literate. Instead, an historical example of the embrace of technological assumptions happened with the Western world, which, heavily influenced by literacy, took its principles of "uniform and continuous and sequential" for the actual meaning of "rational."
McLuhan argues that media are languages, with their own structures and systems of grammar, and that they can be studied as such. He believed that media have effects in that they continually shape and re-shape the ways in which individuals, societies, and cultures perceive and understand the world. In his view, the purpose of media studies is to make visible what is invisible: the effects of media technologies themselves, rather than simply the messages they convey. Media studies therefore, ideally, seeks to identify patterns within a medium and in its interactions with other media. Based on his studies in New Criticism, McLuhan argued that technologies are to words as the surrounding culture is to a poem: the former derive their meaning from the context formed by the latter. Like Harold Innis, McLuhan looked to the broader culture and society within which a medium conveys its messages to identify patterns of the medium's effects.
"Hot" and "cool" media
In the first part of Understanding Media, McLuhan also stated that different media invite different degrees of participation on the part of a person who chooses to consume a medium. Some media, like the movies, were "hot" - that is, they enhance one single sense, in this case vision, in such a manner that a person does not need to exert much effort in filling in the details of a movie image. McLuhan contrasted this with "cool" TV, which he claimed requires more effort on the part of viewer to determine meaning, and comics, which due to their minimal presentation of visual detail require a high degree of effort to fill in details that the cartoonist may have intended to portray. A movie is thus said by McLuhan to be "hot", intensifying one single sense "high definition", demanding a viewer's attention, and a comic book to be "cool" and "low definition", requiring much more conscious participation by the reader to extract value.
"Any hot medium allows of less participation than a cool one, as a lecture makes for less participation than a seminar, and a book for less than a dialogue." 
Hot media usually, but not always, provide complete involvement without considerable stimulus. For example, print occupies visual space, uses visual senses, but can immerse its reader. Hot media favour analytical precision, quantitative analysis and sequential ordering, as they are usually sequential, linear and logical. They emphasize one sense (for example, of sight or sound) over the others. For this reason, hot media also include radio, as well as film, the lecture and photography.
Cool media, on the other hand, are usually, but not always, those that provide little involvement with substantial stimulus. They require more active participation on the part of the user, including the perception of abstract patterning and simultaneous comprehension of all parts. Therefore, according to McLuhan cool media include television, as well as the seminar and cartoons. McLuhan describes the term "cool media" as emerging from jazz and popular music and, in this context, is used to mean "detached." 
This concept appears to force media into binary categories. However, McLuhan's hot and cool exist on a continuum: they are more correctly measured on a scale than as dichotomous terms.