Web services use the request-response method to communicate among applications. For any communication, we need a medium and a common format that can be understood by everyone, in the case of web services medium is the internet and the common format is the XML (Extensible Markup Language) format as every programming language can understand the XML markup language.
A client is the one that requests some service from the server that is known as the service provider. The request is sent through a message which is in common XML format and in response to that request, the service provider will respond with a message in a common format (.i.e. XML).