Effective writing includes audience and purpose. Every single piece of writing you have ever read has been written with two things in mind: audience and purpose. Imagine a mother who leaves a note on the kitchen table for her children, telling them that there's no milk and asking them to buy a pint before she gets home from work. Even a hastily-written note such as this has an audience (the children) and a purpose (to inform and to make a request).
See how well you understand these concepts by trying this English quiz.