Web services allow different software applications to communicate and share resources regardless of operating system or programming language. They use XML and open protocols like HTTP and SOAP. A web service is self-contained, self-describing application component that can be accessed over the web. WSDL describes the web service so clients know how to access it, while SOAP defines the message format for communication. Visual Studio provides tools to test web services by structuring test cases and suites, managing test requests and results, and automating validation.