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.
Beginner:
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
Intermediate:
Applying Design Issues and Patterns in Web Services
Manage State in Web Services
Advanced:
Orchestrate Web Services with BizTalk Server 2002
Cross Language Barriers with SOAP and a Java Web Service
Web Application Security?The Next Evolution
Managed Security for Your Web Services
Classes:
Introduction to ASP.NET
Building Web Services with ASP.NET
XML Web Services One Conferences 2003
Good Links:
W3C.org
Macromedia Designer & Developer Center: Web Services
MSDN: Web Services Center Home
Sun Microsystems: Web Services Resource Center
O’Reilly XML.com: A Web Services Primer
Discussion Groups:
DevX java.web.services Discussion Group
DevX dotnet.web.services Discussion Group
Book:
Web Services Essentials, by Ethan Cerami.