ORM(for Object Relational Mapping) is a programming technique used to convert different types of data to Objects and vice versa.

In Magento, ORM is shown as Model which further breaks down to two types of Models – Simple and EAV.

All Magento Models interacting with database are inherited from Mage_Core_Model_Abstract class, which is further inherited from Varien_Object.

Difference between two Models is, Simple Model is inherited from Mage_Core_Model_Resource_Db_Abstract class,
while EAV is inherited from Mage_Eav_Model_Entity_Abstract.