SOAP is a vocabulary using the namespace http://www.w3.org/2003/05/soap-envelope for encoding messages.
SOAP consists of mainly of three elements:
[soap:]Envelope - the document element that contains an optional [soap:]Header element followed by a required [soap:]Body element.
[soap:]Header - a wrapper for message "header" or "metadata" information elements.
[soap:]Body - a wrapper for the body of a message.
The [soap:]Fault element can be used in the body as a standard way of sending/returning errors.
You can put whatever you want in [soap:]Header and [soap:]Body.