Answer 1

The <div> tag in html is for organize and structure your web page. For example you can use:
<div class="class1"> text, header, footer, paragraph or whatever you want </div>
in your html code to define a part of your code you want to put on a different font color. After that you can use in your css code:
.class1 {background-color: green;}
to change the properties of this particular "div(ision) class1" (here you put it on a green font).
it allow you to create more parts than the basic <section>, <aside>, <nav>,<header>,<footer>. And you can create as many <div>*****************</div> as you want.