How to override core model in magento ?

For example :

We need to override category.php model file of the catalog module

• Catalog module is placed in the core/Mage directory. Now here override the model file of the category.php
• Create the following directory and files in local direcoty
• app/code/local/Mage
• app/code/local/Mage/Catalog
• app/code/local/Mage/Catalog/Model
• app/code/local/Mage/Catalog/Model/category.php
• Now Open category.php file from this path ( core/Mage/Catalog/Model/ )
• Copy the content of this file
• paste copied content in new category file that is placed in this file path( local/Mage/Catalog/Model )
• Now you can add new action or edit existing action in this model file.