SOAP stands for simple object Access protocol .
 
 SOAP structures a message into two main parts: the headers
and the body. 
SOAP is more than just a sender-receiver protocol, although
that, too, is certainly the dominant use today. SOAP
supports the concept of a message passing from a recipient,
possibly through one or more intermediaries, and ending up
at its destination, more precisely known as the ultimate
receiver.

SOAP is core part of WEB-SERVICE. Mainly SOAP is an XML
syntax for exchanging message because of XML is language and
platform
indepent.SOAP is use when we are creating web services
because  Web services means publish a method(method resolve
business logic) globally.

Simple object access protocal