What is a div in HTML?

A div is a container that is used to hold content and design elements.
When given an id or class, it is used in conjunction with css like a building block on the page. You can put divs side-by-side, stack them, even put one in front of another. A div can also be used as a parent
for the elements inside it. So with css, you can say,
make all the text in div x red, make all the text in div
y blue.
Basically, they are the building blocks of a webpage.
Turn on the developer tools in any major web
browser, right click, and select Inspect. This will show
you the elements of the page, and highlight them as
you mouse through the code. It also shows you how
they are styled. An excellent learning tool, and a
major tool in web development.

Source