1. Download the module from http://drupal.org/project/extlink.
2. Extract and copy the module directory to the sites/all/modules directory on your site.
3. Enable the module.
4. Configure the module settings at yourdomain.com/admin/settings/extlink.
5. In the configuration screen you can choose to apply icons to external links and mailto or one of them.
6. You can choose to configure external links in open in a new window and display a confirmation message when leaving the site.
7. You can use regular expressions to filter the links that will be considered external and those that will not.
8. Now if I create a link that points to something outside my site, external links module adds to it an icon.
Finally I can say that this module is a good module for making site interface more users friendly, you may also take a look at External Link Extras, which can expand the functionality of External Links.
I hope you enjoyed this Module Preview and stay tuned for more to come.
Senior Drupal Developer, JesoX LLC