For what purpose <div> tag is used?

Answer 1

div tag is used as a section or container... u can nest other tags in it.

Answer 2

A div (division) tag is used to select an area in your webpage to modify with CSS or to get the ID of for use in JavaScript.
Example

HTML:
<div id="form">
<input type="text">
<button><button>
</div>

CSS:
div#form {
background-color: red;
}

This would make the background red on any division with ID "form".