Expository preaching is a distinct form of preaching that is usually set over and against what is commonly called topical preaching. In topical preaching, the minister begins by thinking of a particular topic he wants to address, such as home-life, fatherhood, salvation, or work). After selecting his topic, he will try and find Scriptures that go along with that topic and weave them into the sermon.
In expository preaching, the text is put first, and the Scripture determines the topic. It may be confessed first that we begin by choosing a book of the Bible to work through (currently, we are in the gospel of Luke). The minister will begin by reading the text and seeking to understand it in its context. He will break the text down, line by line and word by word to determine its meaning.
After scrutinizing that particular text, he will begin to construct his sermon. The theme of the passage under scrutiny will become the topic of the sermon. The points too will be drawn from the text. In sum, the whole message is attempting to explain that particular passage of Scripture
The aim of this expository approach is to "let God speak." While this can certainly be done in topical preaching, it may also be the case that the voice of God is more easily skewed. In topical preaching the Scriptures can easily be pulled out of their context or the pastor can "hobbyhorse" on his particular topics. It is also likely that he will not be as diverse as the Scriptures themselves are in his topical choice.
In expository preaching one can get an in-depth understanding of Scripture because you are examining each and every line, and sometimes every word!. As you work through a book of the Bible, you become familiar with the whole counsel of God because you are sucking the marrow out of every bit of it.
All this is, of course, for the listener's benefit. As each passage of Scripture is opened up, we believe the Holy Spirit has more opportunity to convince, convert, and convict those who are listening.