For most people, though, communication isn't formal, but a way of life. It's the means by which people create and maintain relationships, share knowledge with each other, express our feelings, and otherwise go about participating in a society.
However, that definition is too informal.
Roman Jakobson in his Linguistics and Poetics (1958) devised the following model for communication.
addresser ---------------------------------------------- addressee [emotive] [connative]
Context or referential function. What is being spoken of, what is being referred to. In the expression "Put those on the shelf" the referential burden of the message is "Put (the bags in your hands) on the shelf (over there to your left) (right now)." See WhatIsContext.
Poetic function. The focus on the message (the use of the medium) for its own sake. The associations (equivalence, similarity and dissimilarity, synonymity and antonymity); the repetitions of sound values, stresses, accents; the word and phrase boundaries and relationships (e.g. elided vs end-stopped words): as these are combined in sequence. Related to MediumIsTheMessage?. (IsMediaTechnologyOrContent?)
Emotive or expressive function. Refers to the attitude of the addresser towards that of which (or to whom) he speaks: through emphasis, intonation, loudness, pace, etc. "I'm REALLY serious." See ParaLanguage.
Phatic function. The use of language to keep people in contact with each other, the maintenance of social relationships, including "idle chat", "small talk.". "How 'bout that weather?" Most birds in the wild chirp just to let each other know they are there. This creates a mood or atmosphere of sociability or community.
Metalinguistic function. Language employed to verify the receiver understands what's being said, or the receiver has the same the same codes in the same contexts. "Are you with me on this one?" Also, BodyLanguage, error-correcting codes. Possibly on a separate CommunicationChannel.
Connative function. Those aspects of language which aim to create a certain response in the addressee. "Learn this now!"
See also WhatIsaMedium, WhatIsContext.