An internal link is a multiple links on your own site. That is from one webpage to another page on your site.
You can say that it is link on the same domain. Internal linking also promotes healthy link equity flow throughout a site.
Internal linking is a great way to boost links to important pages on your site and it helps visitors to go through your website too.
This also make the visitor to be on your site for maximum time.
Internal links can be done through many ways. For example you are writing the post on a topic and you have also post related to it. You make a key work for and make it a link. This will also create your internal link.
Many widgets are also there which makes your internal link. You can use LinkWithin as i have made on my site.
Another way to create internal links is to drop links on your home page. You can do it by creating a widget of popular post, labels, tags , blog archive, recently posted, most commented post etc.