A web service is any piece of software that uses REST and/or a standardized XML messaging system, described by a WSDL (Web Service Definition Language) (pronounced "wiz-dal"), to exchange data between applications or systems. A web service must have some simple mechanism for interested parties to locate the service and its public interface using standard network protocols, such as, but not limited to, SOAP over HTTP. Software applications written in various programming languages and running on various platforms can use web services transparently to exchange data over computer networks, like the Internet, because the WSDL serves as the definition of the language used by the computers.
A simple example of a Product Advertising API request uses the ItemSearch operation.
All Product Advertising API requests are based on REST or SOAP and all of the responses are based on XML.