When you have a conversation with someone, there are four elements in play.  There is a speaker, a listener, a message, and a type of language.  The speaker, listener, and message determine the type of language that is used. 

For example, a conversation between friends typically sounds different than a conversation between supervisor and employee.  Friends typically use a casual and informal language while a more formal language is typically used with a supervisor.

The same four elements that apply to conversation apply to writing.  All forms of written communication have a writer, a reader, a message, and a type of language.  However, each of the four purposes focuses on ONE of these four elements.