The Simple Object Access Protocol (SOAP) is a lightweight protocol for exchanging structured and typed information in a decentralized, distributed environment. SOAP 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 datatypes, and a convention for representing remote procedure calls and responses.
SOAP is about interoperability between applications running on disparate platforms using various implementation technologies in various programming languages.
A SOAP message is a kind of XML document. SOAP message is usually carried as the payload of some other network protocol. SOAP messages are independent of any operating system and can potentially be transported using a variety of other protocols, including SMTP, FTP, and HTTP. However, the WS-I Basic Profile 1.0 sanctions the use of SOAP over HTTP.