A navigation Link is a specialized tool to add menu hyperlinks to web pages outside of your website. For example, you might want a link to your sister company in your main site navigation. To add a navigation Link, select "Add" from the Action Bar in the site Panel like you would any other page. Select "Link", and provide a Title and URL. In this case a URL is different than a Path as the Link expects a full URL like "http://www.acme-sister-company.com" and must include "http://".
The Link can then appear in your site menu as long as "Include in Navigation" is selected in Link Settings.