div - Literally means, "division"
The div tag is used to "divide" a web page into different sections. These sections are boxes.
We use the div tag to generate boxes on a web page, and then we use CSS to style these boxes. In these boxes we will place our content.
A Layout is made up of several div tags that are positioned in such a way so as to create a layout of columns and/or rows. Each div is styled with CSS. It is with CSS that we position each div tag.
HTML is designed to work in conjunction with CSS, therefore, we have far fewer HTML tags to learn. This is due largely to the fact that CSS is used for all formatting and positioning. Whereas HTML & XHTML are used to create structures and add content.
Video: What is a div tag?
Note: You will find the div tag in Expression Web's Toolbox Panel (upper right corner by default).

Two Ways to Add a div tag to a web page:
1. Click and Drag the div from the toolbox and drop it on the web page.
2. Or, place your cursor on the page near the location to insert a div, then double click on the in the toolbox.
Regardless of the method you use to insert a tag, the result is the same. Expression Web writes the following code for you: <div></div>
Of course, you can also just write the same code in the code yourself.
See "Rectangular Boxes & Box Generation"