HTML tags are arranged in a hierarchy. This is sometimes called nesting tags or creating an HTML tree. Between the opening <header> tag and the closing </header> tag there are two other tags. We call these child tags, because they have a parent-child relationship.
<div>
<header>
<h1>Title</h1>
<nav>
<a href="#">A link</a>
<a href="#">Another link</a>
</nav>
</header>
<main>
<img src="http://enes.in/f.png" />
<p>Ea commodo consequat duis autem vel eum iriure dolor in hendrerit.</p>
<p>Decima eodem modo typi qui nunc nobis videntur.</p>
<p>Tincidunt ut laoreet dolore magna, aliquam erat volutpat ut wisi enim ad minim.</p>
<p>Id quod mazim, placerat facer possim assum typi non!</p>
<p>Legere me lius quod ii, <a href="#">legunt saepius</a> claritas.</p>
<p>Gothica quam nunc putamus parum claram anteposuerit litterarum formas humanitatis per seacula.</p>
</main>
<footer>
<ul>
<li><a href="#">Foo</a></li>
<li><a href="#">Bar</a></li>
<li><a href="#">Bam</a></li>
<li><a href="#">Baz</a></li>
<li><a href="#">Trek</a></li>
</ul>
</footer>
</div>
