From John Piper:
Expository means that preaching aims to exposit, or explain and apply, the meaning of the Bible. Every sermon explains and applies the Bible. The reason for this is that the Bible is God’s word, inspired, infallible, profitable—all sixty-six books of it. The preacher’s job is to minimize his own opinions and deliver the truth of God. Therefore, it is mainly Bible exposition—explanation and application.
And the preacher’s job is to do that in a way that enables us to see that the points he is making actually come from the Bible. If they come from the Bible and you can’t see that they come from the Bible, your faith will rest on man and not God.
The aim of this exposition is to help you eat and digest some biblical truth that will make your spiritual bones more like steel, and double the capacity of your spiritual lungs, and make the eyes of your heart dazzled with God’s greatness, and awaken the capability of your soul for kinds of spiritual enjoyment you didn’t even know existed.
Preaching is also exultation—expository exultation. This means that the preacher does not just explain what’s in the Bible, and the people do not simply understand what he explains, but the preacher and the people exult over what is in the Bible as it is being explained and applied.
Preaching As Worship
Preaching does not come after worship in the order of the service. Preaching is worship. My job is not done if I only see truth and show it to you. The devil could do that—for his own devious reasons. My job is to see the glory of the truth and to savor it and exult over it as I explain it to you and apply it for you. That’s one of the differences between a lecture and a sermon.