Site aliases that reference remote websites can be accessed by Drush through a password-less SSH connection (http://en.wikipedia.org/wiki/Secure_Shell). Before we start with these, let's make sure that we meet the requirements.
First, it is recommended to install the same version of Drush in all the servers that host your website. Drush will fail to run a command if it is not installed in the remote machine except for core-rsync, which runs rsync, a non-Drush command that is available in Unix-like systems.
If you can already access the server that hosts your Drupal project through a public key, then skip to the next section. If not, you can either use the pushkey command from Drush extras (https://www.drupal.org/project/drush_extras), or continue reading to set it up manually.