...maybe we can agree that almost every word in a sentence can be categorized as either a content word or a functional word. The content words comprise the nouns, adjectives, adverbs, and most verbs: they are carriers of information and suppliers of sensory evidence. The functional words are the prepositions, the conjunctions, the articles, the toof an infinitive, and such—the kinds of words necessary to hold the content words in place on the page, to absorb them into the syntax. The functional words in fact tend to recede into the sentence structure; their visibility and audibility are limited. It’s the content words that impress themselves upon the eye and the ear, so the writer’s attention to sound and shape has to be lavished on the exposed words. They stand out in relief.
The Sentence Is a Lonely Place Gary Lutz. A lecture delivered to Columbia University's Writing Program in 2008.