PayPal is a Web-based application for the secure transfer of funds between member accounts. It doesn't cost the user anything to join PayPal or to send money through the service, but there is a fee structure in place for those members who wish to receive money. PayPal relies on the existing infrastructure used by financial institutions and credit card companies and uses advanced fraud prevention technologies to enhance the security of transactions.

Integration API
 
The PayPal Integration Center contains all the resources and support you need to build PayPal payments into an e-commerce website or application. The PayPal API makes powerful functionality available to developers by exposing various features of the PayPal platform. To use the PayPal API, you must have a Premier or Business account, and you must acquire a set of API Credentials to authenticate your API calls. PayPal offers APIs for sending money, receiving money, refunding transactions, searching transaction histories, and more.
 

• Express Checkout : Use the PayPal API to provide the quickest, easiest checkout experience. You can also use Express Checkout to set up recurring payments.

• Website Payments Standard : Use basic HTML to accept credit card, bank account, and PayPal payments.

• Website Payments Pro : Get our most comprehensive solution to process credit cards directly on your site and also accept PayPal and bank account payments with Express Checkout.

• PayPal Mobile Checkout : Get our straightforward merchant interface to enable a quick, hosted buyer checkout flow on mobile devices.

• Payflow Link : Use basic HTML to accept credit cards with this simple hosted solution using the Payflow payment gateway.