An internal link in HTML is a link to some other section of the same document. By clicking an internal link you jump to other part of the same web page. For example when you at at the bottom of a lengthy web page, you can create an internal link at the bottom to jump at the top without the need to scroll. To create an internal link you first need to mark the target area with an anchor tag like this
<a name="top"></a><!-- this line is marked as a pointer, clicking Top link will scroll your page up here -->
At the bottom of your page, you can add a link to jump at the top
<a href="#top">Go to Top</a>
A
Demo of internal links better explains this.