SOAP is a protocol for the exchange of information in a distributed environment. SOAP messages are encoded as XML documents and can be exchanged using various underlying protocols.
The SOAP specifications describe a distributed processing model in which a SOAP message is passed between SOAP nodes. The message originates at a SOAP sender and is sent to a SOAP receiver. Between the sender and the receiver, the message might be processed by one or more SOAP intermediaries.
A SOAP message is a one-way transmission between SOAP nodes, from a SOAP sender to a SOAP receiver, but messages can be combined to construct more complex interactions, such as request and response, and peer-to-peer conversations.