Context-Based URI Model Loading

Now that we're in our Action method entry point, we'll want to start instantiating classes that do things. Magento offers a special way to instantiate Models, Helpers and Blocks using static factory methods on the global Mage class. For example:

Mage::getModel('catalog/product');
Mage::helper('catalog/product');

The string 'catalog/product' is called a Grouped Class Name. It's also often called a URI. The first portion of any Grouped Class Name (in this case, catalog), is used to lookup which Module the class resides in. The second portion ('product' above) is used to determine which class should be loaded.
So, in both of the examples above, 'catalog' resolves to the Module app/code/core/Mage/Catalog.
Meaning our class name will start with Mage_Catalog.
Then, product is added to get the final class name

Mage::getModel('catalog/product');
Mage_Catalog_Model_Product

Mage::helper('catalog/product');
Mage_Catalog_Helper_Product

These rules are bound by what's been setup in each Module's config file. When you create your own custom Module, you'll have your own grouped classnames (also calles classgroups) to work with Mage::getModel('myspecialprefix/modelname');.
You don't have to use Grouped Class Names to instantiate your classes. However, as we'll learn later, there are certain advantages to doing so.