An application programmatic interface (API) is the interface that allows exchange of data between applications or systems. Using APIs, it helps you to integrate seemlessly into service provider's website to acquire their data/services at real time. There are various service providers across the world that provides free/paid API to integrate and expand their services. A Web service interface is to be created that acts like a web form to pass the information into a web service and display the results in XML format.

Below are the list of sites that offers APIs for specific purposes

• Ecommerce API - Froogle, Shopping.com, Amazon
• Logistics / Shipping API : Fedex, UPS, USPS, DHL, Airborne and many more
• Affiliates API- Clicksgalore, Commision Junction & many more
• AdNetworks API - Google Adwords and Adsense API
• Others APIs - Cheque Verification / Clearance, Online Reservations, Stock Quotes, Charts, Weather etc
• Maps Integration API- Google API, Yahoo API, Mapquest API