Layer Naming - Joomla and Gimp/PS

I found this, on the website and was wondering if people could specifically tell me what each layer does.

* content_joomla - this is the most important one. This is where most of the regular Joomla content gets rendered by default.
* breadcrumb_joomla
* left_joomla
* right_joomla
* top_joomla
* user1_joomla
* user2_joomla
* user3_joomla
* user4_joomla
* footer_joomla
* debug_joomla
* syndicate_joomla

Thanks in advanced.