What Is The Difference Between Payment Method And Payment Classes?

Q. What is the difference between payment method and payment classes (such as order_payment, quote_payment, etc.)?

Payment model is the abstract of an instance payment method and it works with database. Payment model can be Mage_Sales_Model_Order_Payment or Mage_Sales_Model_Quote_Payment depending on checkout stage. And the database is stored corresponding with tables sales_flat_order_payment and sales_flat_quote_payment. However, Quote_Payment doesn’t have any method to process the order.