Cache servers depend on cache control headers provided by the web server. Essentially, the web server (based on the configuration) specify’s what content is cache-able and for how long. (Note: Some of the cache servers might ignore this and have a default cache period for specific content. But that is not for another post 🙂 )
Options FollowSymLinks MultiViews
Allow from all
ExpiresDefault "modification plus 1 hour"
This configuration tells apache to enable cache headers for all content in the /var/www/html/static folder. The cache expiration is set to expire 1 hour from the modification time of the content.