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:
Everyday or formal?
Poetic or straightforward?
Descriptive or clear?
Tone refers to the mood or feel of your writing, eg should your tone be:
Friendly and chatty, or distant and polite?
Sarcastic, pleading, or dismissive?
Upbeat and positive, or calm and controlled?
Structure refers to the way you present your writing, eg should you use:
Full sentences and paragraphs?
Subheadings and bullet points?
A sequence of linked paragraphs that lead to a final conclusion?