<div> tag defines a division/section/area. usually, it would be used to seperate posts, use it in some cases to make different color charts. the most basic example is

<div style="color:#00FF00">
  <h3>This is a header</h3>
  <p>This is a paragraph.</p>

</div>

this makes the selected text the input color.

<p> ,as shown above, is paragraph. it is where you write everything under the header.