Adding a subtitle to the main title
As @Smalution pointed out, this is generally done using the site Slogan option which is set at /admin/config/system/site-information.

This is the standard way of doing this but relies on the theme you are using have the line 

&lt;?php print $site_slogan; ?&gt;


Some where in the page.tpl.php

If you are creating your own theme, or hacking some one elses make sure this is in there. But, in some themes (this goes for Garland that ships with D7) you will see something like 

&lt;?php print $site_name_and_slogan ?&gt;


in the page.tpl.

This is because the page template has gone through a preprocess function in the template.php i.e function garland_preprocess_page(&amp;$vars) and the slogan has been wrapped with the title and a new variable created :

$vars['site_name_and_slogan'] = $site_name_text . ' ' . $slogan_text;


So if you can't see $site_slogan straight away that's probably why.