SOAP is part of a web services 'stack'

Transport Protocol	Typically HTTP
Services Interface	SOAP
Service Description	WSDL
Service Publishing	UDDI

The larger system that SOAP plays a role in is a fairly comprehensive set of protocols for discovering, describing, consuming and exchanging XML payloads. We won't get into the details of the discovery mechanism (UDDI) and only get into the description mechanism (WSDL) enough to be consumers of SOAP services.