Magento – use of getFacetedData() in Magento 2

I would say in magento 1.8 all collections are resource collections.

Resource collections depend on generic implementations for all the SQL-stuff. the idea is to only reimplement the base class to offer support for another sql database, like postgre, oracle or mssql.

The still existing Mysql4 classes are only for backwards compatibility but only extend the resource collection

Update:

Could you tell me what are the three type of models in magento ?

I'm not sure about this, but I would say you are maybe talking about Models (extending Mage_Core_Model_Abstract), Resource Models (extending Mage_Core_Model_Resource_Db_Abstract) and Collections (extending Mage_Core_Model_Resource_Db_Collection_Abstract).

You cannot use group directly from the collection.
But each collection has an instance of Zend_Db_Select associated to it. This is the actual query that is going to be executed when the collection is loaded.
So you can use:

$collection = ....
$collection->getSelect()->group('group_field_here');