SOAP was originally an acronym for Simple Object Access Protocol. This acronym was dropped from SOAP Version 1.2 and now it is a communication protocol to exchange structured information in a decentralized, distributed environment. The SOAP protocol exclusively relies on XML, and all its messages are in the XML format. It was originally developed by Microsoft and became a W3C recommendation in 2003.
REST is an acronym for Representational State Transfer. It is an architectural style to retrieve/update resources within a distributed computer system. It relies on a stateless, client-server, cacheable communications protocol, and in virtually all cases, the HTTP protocol is used. REST was developed by W3C in parallel with HTTP 1.1, based on the existing design of HTTP 1.0.
SOAP is a resource-heavy choice to build services but it has the following advantages over REST:
It is transport independent (REST supports HTTP only)
It supports all WS-* standards (REST does not)