Which Word Should We Choose, Formal or Informal?
Just as there is formal and informal dress, so there is formal and informal speech. One is not supposed to turn up at a ministerial reception or at a scientific symposium wearing a pair of brightly coloured pyjamas. (Jeans are scarcely suitable for such occasions either, though this may be a matter of opinion.) Consequently, the social context in which the communication is taking place determines both the mode of dress and the modes of speech. When placed in different situations, people instinctively choose different kinds of words and structures to express their thoughts. The suitability or unsuitability of a word for each particular situation depends on its stylistic characteristics or, in other words, on the functional style it represents.
The term functional style is generally accepted in modern linguistics. Professor I. V. Arnold defines it as "a system of expressive means peculiar to a specific sphere of communication". 
By the sphere of communication we mean the circumstances attending the process of speech in each particular case: professional communication, a lecture, an informal talk, a formal letter, an intimate letter, a speech in court, etc.
All these circumstances or situations can be roughly classified into two types: formal (a lecture, a speech in court, an official letter, professional communication) and informal (an informal talk, an intimate letter).