The User Interface is implemented as a large set of Smarty templates, which are called from the various Page classes. (See the section titled “Request Handling”.)
These templates are responsible for the HTML markup of each page; however, all content is provided either by template variables (such as article titles) or through locale-specific translations using a custom Smarty function.
You should be familiar with Smarty templates before working with OJS 2.x templates. Smarty documentation is available from http://smarty.php.net.