(1) (noun) Any data transmitted over a network that is intended for human interpretation.  Just as a program becomes a "job" or "process" when it runs in the computer, data becomes a "message" when it is transmitted from one person to another.  Email, text and chat are examples.  See 
communications protocol, 
email, 
text messaging and 
instant messaging.
(2) (verb) To send a message.  For example, "message me" means send me a text or instant message.  See 
text messaging and 
instant messaging.
(3) In object technology, communicating between objects, similar to a function call in traditional programming.  See 
object-oriented programming.