Message is defined as the physical manifestation of information as produced by the source
There are two distinct message categories:Analog
An analog message is a physical quantity that varies with time usually in a smooth and continuous fashion.
Examples of analog messages are:(i) the acoustic pressure produced when you speak
(ii) the angular position of an aircraft gyro(iii) the light intensity at some point in a television image.
Digital
A digital message is an ordered sequence of symbols selected from a finite set of discrete elements.
Examples of digital messages are:(i) letters printed on this page
(ii) listing of hourly temperature readings(iii) the keys you press at a computer terminal.
Whether the message is analog or digital, it needs to be converted into an electrical signal. (There are only few messages, which are inherently electrical.)