Config files

Module configuration files are found in the etc folder under a module’s main directory. There are 3 different config files available, all of which are XML.

. config.xml

. system.xml

. convert.xml

The only config file that directly affects your module’s behavior is config.xml. The other two, system.xml and convert.xml, automatically create some setting forms for you on the system’smain backend configuration page.
The contents of allmodules’ config files are merged into one massive collection of settings. This means that you can override the settings of any module in any other module simply by putting in the correct XML tags. This is the essence of overriding inMagento.

You can create any class for any purpose, and to install it into the system you create a new config.xml that specifies your class name in the same spot where the original class was defined. This is also why you will see method calls like getModel(’catalog/product’) used throughout the system instead of a more simple approach like: new Catalog_Model_Product().
The use of “tags”, or names, for each class gives you a powerful way to override any part of the system.