SOAP or Simple Objects Access Protocol is a web communication protocol for exchanging structured information in a decentralized, distributed environment. 

APIs (application program interfaces) that comply with the principles of SOAP enable XML messaging between systems through HTTP or Simple Mail Transfer Protocol (SMTP) for transferring mail.

SOAP, unlike the REST pattern, supports the XML data format only and strongly follows preset standards such as messaging structure, a set of encoding rules, and a convention for providing procedure requests and responses.SOAP is mostly used with enterprise web-based software to ensure high security of transmitted data. SOAP APIs are preferred among providers of payment gateways, identity management and CRM solutions, as well as financial and telecommunication services.