A payment gateway is an e-commerce application service provider
service that authorizes payments for e-businesses, online retailers,
bricks and clicks, or traditional brick and mortar. It is the equivalent of
a physical point of sale terminal located in most retail outlets.
Payment gateways encrypt sensitive information, such as credit card
numbers, to ensure that information passes securely between the
customer and the merchant.

Google Checkout is an online payment
processing service provided by Google
aimed at simplifying the process of
paying for online purchases. Users
store their credit or debit card and
shipping information in their Google Account, so that they can
purchase at participating stores at the click of a button. Credit cards
accepted are VISA, Mastercard, Discover, American Express and more.
Google Checkout also offers fraud protection, as well a unified page
for tracking purchases and their status.
 Google Checkout service was first made available in the United
States on June 28, 2006. The service later became available in the
UK. Google Checkout was free for merchants until February 1, 2008,
Since that date Google charges merchants 2.0% plus $0.20 per
transaction (1.5% + £0.15 for UK merchants). Also since this date,
merchants who advertise with an Adwords account will not be
charged fees on monthly transactions totaling less than ten times
their monthly AdWords expenditure.

PayPal performs payment processing for online
vendors, auction sites, and other commercial
users, for which it charges a fee. It sometimes
also charges a transaction fee for receiving
money (a percentage of the amount sent plus an
additional fixed amount). The fees charged depend on the currency
used, the payment option used, the country of the sender, the country
of the recipient, the amount sent and the recipient's account type.
Generally, they range from 1.9% - 2.9% + $0.30 per transaction.

Security key

In early 2007, PayPal introduced an optional security key as an additional precaution against fraud. A user account tied to a security key has a modified login process: the account holder enters their login ID and password, as normal, but is then prompted to press the button on the security key and enter the six-digit number generated by it. This two-factor authentication is intended to prevent an account from being compromised by a malicious third party without access to the physical security key.