The first week of school was extremely hot, but I made it through without a problem. We were assigned to write two different letters. One to a friend, and one to the editor of Eagle News. Depending on your audience, it changes your tone and writing style.
One of the first assignments in class was to write an informal letter to your best friend. The guidelines were to free write a letter about the first week of school, as if you were really writing them a note. When the audience is a friend/peer you are more likely to just write in a style that is the quickest and most attractive to you. When I write I don't like to have many grammatical errors, or use slang.
The letter to the editor wasn't much different to my letter to Rebecca, except for the fact I didn't care to have a response from the editor. I didn't use proper punctuation throughout the letter to Rebecca, but I did when I wrote to the editor.
Depending on your audience and purpose, it can change the whole style and point trying to be made about the piece.