Please expain <div "class" > and class "selected"

Answer 1

The class is define to style some elements in your page, then you can style all the elements with the same class by using selector in css:
.myclass {color:blue;}

Answer 2

The class is used so you can define a certain object or element in your code with CSS. For example, if you wrote in HTML:
<div class="hi">
In CSS to style it, you would do something like this:
.hi{
font-family:"Sans Serif",cursive;
font-size:22px;
}
This code now has a class called "hi" that is in Sans Serif font and has writing that is 22px large.