Learning how to use Web services takes for granted a solid foundation in XML, mainly because the attendant technologies used in Web services are XML-based (such as SOAP and WDDI). Most of these articles focus on Web services implementations either in Java or .NET.
What Are Web Services?Building Web Services with .NET
Start Writing Web Services Today (Java)
Special Report: Winning with Web Services (Java)
Converting Web Pages into XML Data Sources
Web Services and UDDI: The New Wave of E-Business Renaissance
Closing the Loop in Web Services
Web Services Essentials, by Ethan Cerami.