Use of $installer v $this in install scripts

$installer = $this;
you can uses instance of $this is 

$installer = new Mage_Eav_Model_Entity_Setup; 

or

$installer =new Mage_Catalog_Model_Resource_Setup;

$installer = new Mage_Sales_Model_Resource_Setup('core_setup');   
$installer->startSetup();

$installer->addAttribute("quote_address", "prepaiddiscount_total", array("type"=>"varchar"));

$installer->addAttribute("order", "prepaiddiscount_total", array("type"=>"varchar"));

$installer->endSetup();