The simple object access protocol (SOAP)[12] is a lightweight protocol for exchange of information in a decentralized, distributed environment. It is an XML-based protocol that consists of three parts: an envelope that defines a framework for describing what is in a message and how to process it, a set of encoding rules for expressing instances of application-defined data types, and a convention for representing remote procedure calls and responses.
Plain language version: You own an Internet-connected soft drink machine. Once a day, you want your soft drink machine to send information to the inventory management package. You then want the inventory management package to talk to your payroll system so that the salesman for the territory gets paid the right amount. Either you buy all these systems from the same vendor, or you buy each one from the lowest bidder and make sure they all talk SOAP to one another. You then only need one programmer to make it all work instead of a staff of fifteen programmers to write it from scratch.
Although usually associated with Microsoft, it is actually maintained by a consortium of companies, of which Microsoft is one.