The following quote is a well-written description of expository preaching that I found in a book review article.
At the same time, evangelical preachers will want to take note of Packer’s strong words of caution (which have a wider application than just to the preacher). He assumes the content of preaching will come from Scripture, and insists a preacher know it and instill in his congregation a desire to learn it; the preacher, after all, is “there to teach from the Bible.” Packer, then, is critical of preaching that promotes anything besides Scripture as central. He prefers expository preaching—preaching that exposes the text. In topical preaching, on the other hand, preachers “take a topic, they appeal to one or two Scriptures to illustrate the topic, they tell stories from their experience and other people’s experience to illustrate the topic.” The focus of a topical sermon therefore shifts constantly from Bible to preacher—and the authority by which a listener is expected to believe and obey is “the human authority of a knowledgeable person,” and nothing more.