Accessing data on Drupal with external application
Yes, you can use module Views to create content listings (http://drupal.org/project/views) with Services (https://drupal.org/project/services) and combine them to web services with the content listings using Services Views module (https://drupal.org/project/services_views).

Also, look at Drupal iOS SDK (https://github.com/kylebrowning/drupal-ios-sdk) for some basic already implemented connectors from ios to drupal. (Retrieving specific single user/piece of content, retrieving already mentioned views etc...)

I would start with downloading and enabling the modules, understanding views (https://drupal.org/node/1911062), then read the documentation for services (contained in the module)