One distinction that is sometimes made is between lexical categories and functional categories.
Lexical categories contain the content words--nouns, verbs, etc. These are the words that carry the primary meaning of the sentence. There are also words that carry little specific meaning of their own. Their main purpose is to serve as the glue to hold the content words together. Such words belong to functional categories. Although this distinction is conceptually useful, it's not always easy to assign categories clearly to one group or the other. Prepositions, as we will see, have some lexical qualities and some functional qualities.
For that reason, we will not make too much of the lexical vs. functional distinction. Instead, we will simply describe the primary categories. We will examine how these categories work in more detail as we learn more about sentence structure. For now, here's a brief overview.