Service Oriented Architecture (SOA) makes software quality both more important and more difficult to achieve.
SOA test is "a comprehensive testing and analysis tool suite for tailored to the unique testing and validation needs of Service Oriented Architectures." It helps testers test the message layer functionality of their services by automating their testing and supports numerous transport protocols including: HTTP 1.0, HTTP/1.1, JMS, MQ, RMI, SMTP, TIBCO, .NET WCF HTTP, .NET WCF TCP. Basic testing functionality include static analysis through WSDL testing, functional unit testing, regression testing, security testing, and load testing. SOA test introduced application behavior virtualization, which creates copies of applications or back-end systems that are otherwise difficult to access or exercise during development and testing
A service-oriented architecture (SOA) is the underlying structure supporting communications between services. SOA defines how two computing entities, such as programs, interact in such a way as to enable one entity to perform a unit of work on behalf of another entity. Service interactions are defined using a description language. Each interaction is self-contained and loosely coupled, so that each interaction is independent of any other interaction.
Simple Object Access Protocol (SOAP)-based Web services are becoming the most common implementation of SOA.
Now days we see lots many applications (mobile, web) which are driven by APIs. SOA testing is conducted for all the products which are based on APIs. You can find more details about SOA testing on Wikipedia.