SOAP (Simple Object Access Protocol) is a protocol specification for exchanging structured information in the implementation of Web Services in computer networks.
Basically all web service request and response comes under this category i.e. uses SOAP. Now coming to the question. Being a user or tester we could only see XML requests and response even though Web Services use SOA Protocol.
1) XML Testing In XML Testing we check for proper schema of the XML
SOAP Testing Here in SOAP testing we do a schema test for response that we get from a Web Service
2) XML Testing Validate the tag values as per specification
SOAP Testing Validate the tag values as per specification and u can do additional validation and check it in Data base if u have access
3) XML Testing Its basically static test we do on a static XML Files Such as Config Properties files
SOAP Testing Its dynamic test our response will be based on request that we pass