An absolute link is used when the href has a link that includes the entire domain name. A relative link is a path to a resource relative to the current HTML document’s location, according to how the web server is determining that location. Most often, an HTML document’s location is determined by its directory.
If I wanted to refer someone to a website, I would use absolute links. If I wanted to refer to links on the same server as the page that contains them, I would use relative links.
Hope this helps :)