What is <div>

Answer 1

The <div> tag defines a division or a section in an HTML document.

The <div> tag is used to group block-elements to format them with CSS.

Answer 2

<divstyle="color:#0000FF">
  <h3>This is a heading</h3>
  <p>This is a paragraph.</p>
</div>

Answer 3

It's a "block" of content in HTML, you use it to structure your page but since HTML5, you can use more reavelant name such as

<aside> <footer> <header> <article>

your code is more readable this way.