Paypal is one of the commonly used payment gateways. They provide a low cost option for payment of goods or services. Creating an account is simple and fast. To become a verified paypal user you will need to validate your bank account. Paypal makes a couple of deposits to your account and you need to confirm the amounts to get validated. Once validated you can accept any credit card on your account.

Instant Payment Notification (IPN) is PayPal's interface for handling real-time purchase confirmation and server-to-server communications. IPN delivers immediate notification and confirmation of PayPal payments you receive and provides status and additional data on pending, cancelled, or failed transactions.

Resources

Paypal

Video

In this video you will see how to enable IPN with paypal and your Drupal CMS with ecommerce module enabled