The CSS box model allows web pages to be laid out using boxes or containers. Each box has content, padding, borders, and margins. The <div> tag is used to define boxes and reserve space on the page. Box dimensions can be specified in pixels or percentages. Padding, borders, and margins are added to the defined width and height to calculate the total box size. Margins can be set to "auto" to center a <div> horizontally on the page regardless of screen width.