When you establish your audience, purpose and form, you can make sensible choices about language, tone and structure.
Audience refers to the reader(s), eg could be an individual, in the case of a text message, or a broader group of people in the case of a magazine article or blog.
Purpose refers to the reason for the writing, eg to persuade the reader, to entertain them, to share information, to warn, to advise, to explain.
Form refers to how and where the writing will appear, eg an email, a blog, a recipe, a leaflet, an article.
Language simply means the sorts of words you use, eg should your language be:
Tone refers to the mood or feel of your writing, eg should your tone be:
Structure refers to the way you present your writing, eg should you use: