An application programming interface (API) is the interface that a computer system, library or application provides in order to allow requests for services to be made of it by other computer programs, and/or to allow data to be exchanged between them.

Normally, users buy and sell items using the eBay online interface, interacting with eBay directly. But with the eBay API, you communicate directly with the eBay database in XML format. By using the API, your application can provide a custom interface, functionality and specialized operations not otherwise afforded by the eBay interface.

If you have built your own custom shopping cart or are using one not integrated with Website Payments Pro, you can use Direct Payment API for credit card processing on your site and Express Checkout to enable a fast checkout for PayPal users

Amazon Web Services provides developers with direct access to Amazon's robust technology platform. Build on Amazon's suite of web services to enable and enhance your applications. We innovate for you, so that you can innovate for your customers. Browse developer innovations in our Solutions Catalog to see the possibilities!

Intuit provides SDKs for several of its products. The following table lists each of these products and the SDK that can be used to integrate with each product.(Quick Books, Quicken)

Authorize.Net's recommended connection method and offers the most secure and flexible integration. AIM allows merchants to host their own secure payment form and send transactions to the payment gateway using an end-to-end secure sockets layer (SSL) connection. AIM is the required connection method for shopping cart developers participating in the Authorize.Net

Embed Google Maps in your own web pages, Write simple mini-apps for the Google homepage and Google Desktop, Read and write data using a simple, standard protocol, Offer a simple checkout flow and securely process online transactions, Put a Google AJAX Search module on your web pages.

Developers can now build apps for My Yahoo!, using the new Yahoo! Application Platform. More than 40 million highly-engaged users worldwide depend on My Yahoo! to organize their lives, feed their daily information needs, and indulge their guilty pleasures. Learn more about developing and promoting relevant and compelling apps for the #1 personalized homepage on the Web.

The MapQuest OpenAPI is a free service that allows you to use JavaScript to easily integrate routing, geocoding and mapping into your website. With this feature-rich API you can create dynamic routing and mapping applications for personal or community use.