What is a Payment Gateway?
A payment gateway is a third party application service provider that authorises and possesses online payments for businesses e.g. online retailers. Payment gateways protect credit cards details by encrypting sensitive information, such as credit card numbers, to ensure that information passes securely between the customer and the merchant and also between merchant and payment processor. The bottom line is they do all the work of getting the money from the customer into your bank account.
There are many Payment Gateway providers such as PayPal, Authorise.net, 2Checkout etc.