SOA stands for Service-oriented architecture. SOA explains it well.
Down the page, you can see -
Analogously, the services themselves are typically described by WSDL, and communications protocols by SOAP. Whether these description languages are the best possible for the job, and whether they will remain the favorites in the future, is an open question.
So, as you can see, SOAP is one communications protocol which is used to implement SOA.
William Butler Yeats: All life is a preparation for something that probably will never happen. Unless you make it happen.