Style Is Important, Too
Have you ever noticed that as you read something written by a particular author, you get familiar with the writing style? As you move from one book to the next, you notice similarities between them. Style is something that is peculiar to each individual writer. Stephen King, for example, writes in a very different style from Janet Evanovich, and not just because each author tends to write about different subjects. Style is an amorphous thing that is attributed to a writer; many writers don't even realize that they have a particular style. They just write.
Bending the Rules
Many beginning writers think that they can bend the rules or do whatever they like, and call it their “style.” As you gain writing success and get more familiar with the rules, you're allowed certain liberties. As you become more adept at the rules, you also gain a better understanding of where you can bend them and still get away with it. So in your early days, be attentive to the way things are done, and worry about breaking the mold later.
If you're writing a quick note to a friend, you're bound to use much different language than you would if you were, say, writing a letter to a lawyer. Tone, diction, and style vary between formal and informal writing. Don't be too formal with your friends, and don't be too informal otherwise.
But style is much more than sticking to the basic rules. Style also encompasses how you put the words together into sentences, how you put your sentences together into paragraphs, and so on. As you develop your writing style, it will turn into your personal “voice.” The way you transform your thoughts into words will become almost habitual and completely natural to you.
Informal Versus Formal Writing
The rules for informal writing are more relaxed. You can get away with slang. You can get away with saying things like, “You can get away with slang.”
Formal writing is often used in academic situations where the audience has a professional background, although it also exists outside the purely academic field. Law and science are two particularly apt examples.
You must be careful not to sound too presumptuous when writing formally; just because a setting is formal does not mean that you should use only big words. You can use nice small words, and nice short sentences, too, but you should not use colloquialisms or other informal conventions. Stick to traditional punctuation styles and grammar usage.