After you insert a div tag, you can manipulate it or add content to it.
When you assign borders to div tags, or when you have CSS Layout Outlines selected, they have visible borders. (CSS Layout Outlines is selected by default in the View > Visual Aids menu.) When you move the pointer over a div tag, Dreamweaver highlights the tag. You can change the highlight color or disable highlighting.
When you select a div tag, you can view and edit rules for it in the CSS Styles panel. You can also add content to the div tag by simply placing your insertion point inside the div tag, and then adding content just as you would add content to a page.