What is SOAP, and what does a SOAP message look like?

SOAP (Simple Object Access Protocol) is a W3C-standardised XML messaging protocol for exchanging data and making remote procedure calls between systems; every message is an Envelope with an optional Header and a mandatory Body.

<?xml version="1.0"?>
<s:Envelope xmlns:s="http://www.w3.org/2003/05/soap-envelope">
  <s:Header> ... security tokens, routing, transaction info ... </s:Header>
  <s:Body>   ... the actual request or response payload ...    </s:Body>
</s:Envelope>

The split is the useful idea: the Body carries the business payload (for example a SAML <ArtifactResolve> request), while the Header carries metadata about the message, such as a signed SAML assertion that identifies the caller. SOAP is usually sent over HTTP POST but is transport-independent.

Why it matters for SAML: the SOAP binding is how SAML parties talk directly, server to server (the back channel), for example to resolve an artifact or to send a logout request without involving the browser.

Go deeper:

• SOAP (Wikipedia) — history, message structure and the comparison with REST.
• W3C: SOAP 1.2 Part 1, Messaging Framework — the normative definition of envelope, header and body.