SOAP, or the Simple Object Access Protocol, is a protocol designed to help Web applications send messages to each other. SIMPLE: It's got a very simple, portable structure, based on XML, and it can travel via HTTP, so it's lightweight and easy to implement, plus it's independent of language and platform. And thus the "Simple" part of the acronym,

One java application needs to convert any European Economic Community currency to euros, and vice versa, at the current market rate. The java program can send a brief SOAP request to a server with a php script that does the calculation and returns a response, also in SOAP. As long as they both use SOAP, the transaction is perfectly clean and simple. Also, if your application is running on a computer with Web access, the SOAP messages can be sent via HTTP, through port 80. Using port 80 is nice because it doesn't require any extra setup, and any firewall sitting in the way assumes it's just more Web traffic, and lets it through, without the long lines and cavity searches that often crop up if you set aside a dedicated port. Three, SOAP is an open protocol (consult the W3C's Working Draft of the SOAP spec for more). There are a number of freely downloadable SOAP client and server tools and libraries that can be bent to your needs.