Defining 'the Medium is the Message'
In 1964, communications theorist, Marshall McLuhan, coined his well-known phrase 'the medium is the message' (Understanding Media: the extensions of Man). But what does this mean? 
 In order to gain a full understanding, it is important to first define the term 'medium' within McLuhan's paradigm as it varies from the customary usage in the arts, namely "the liquid substance (such as oil and water) with which pigments are mixed and applied" (Lütticken 2004) applied" (Lütticken 2004) and the conventional usage referring to the mass-media of communication. Instead, 'medium' is an extension of ourselves either mental or physical. For example, a hammer extends an arm; a book is an extension of the eye. The medium "enables us to do more than our bodies could do on their own" (Federman 2004).  Secondly, the term 'message' has to be clarified within McLuhan's phrase, since it refers not to the content of information as one would assume but to "the change of the scale or pace or pattern that it introduces into human affairs" (McLuhan 1964:8). In other words, the effect of any medium on a person. For McLuhan, this change or effect was of importance, being the character of the medium and not the 'content' of a medium because he felt that the content "blinds us to the character of the medium"(McLuhan 1964:9).
 It is the medium (the extension of ourselves that alters the way one thinks and the way the world is seen. Thus is the form of a particular medium more important for McLuhan than the content conveying a message. Through the concept of 'the medium is the message' McLuhan argues that it is the medium with which one could understand and identify "the psychic and social consequences of the designs or patterns as they amplify or accelerate existing processes" (McLuhan 1964:8). In other words, when the medium is seen as the message it reveals the change or effect of that medium. 
__________________________________________________________________________________
Reference

McLuhan, Marshall, Understanding Media: the extensions of man, United Kingdom (Routledge) 1964.
Lütticken, Sven, 'Undead Media', Afterimage (2004) January 1st, n.p.
Federman, M., 'What is the Meaning of the Medium is the Message?' (2004) July 23, n.p.