there are a few that aren't really supposed to be used in formal writing. contractions, for example, are supposed to be written out--"don't" becomes "do not"--because contractions are something we vocally combined to make our speech a little lazier and flowy. this makes contractions a more "casual" part of speech. interjections aren't encouraged either, because we typically make interjections in casual speech; these don't go hand in hand with formality.
