SOAP defines the format of a message exchange between the web service provider and the consumer:

The top element in a SOAP Message is SOAP Envelope. It contains a SOAP Header (Optional) and a SOAP Body. The SOAP Body actually contains the message payload (for processing by the consumer) and optionally SOAP Fault (Optional), if there is any error.

The SOAP header provides extensibility to the SOAP message. It can contain information such as user credentials, transaction management, and message routing.