I would like to learn how to use the Joomla!Platform, wich comes sepparately from the CMS since J1.6.

I've had some experience with PHP frameworks like CodeIgniter, but I'm not sure what the purpose of Joomla!Platform is. I guess Joomla!Platform is just similar to another PHP frameworks (like CodeIgniter or Symfony), but the Joomla!Platform includes some special libraries used to build the Joomla CMS. Is that right? I would like to grab some beginners' guide for Joomla!Platform.

It would be nice to get some idea about how to develop a simple web application from the Joomla!Platform, a sort of tutorial.