[ Definition: A Web service is a software system designed to support interoperable machine-to-machine interaction over a network. It has an interface described in a machine-processable format (specifically WSDL). Other systems interact with the Web service in a manner prescribed by its description using SOAP messages, typically conveyed using HTTP with an XML serialization in conjunction with other Web-related standards.]

Specs

IBM's Web Services spec page: http://www-106.ibm.com/developerworks/views/webservices/standards.jsp
Microsoft's: http://msdn.microsoft.com/library/en-us/dnglobspec/html/wsspecsover.asp

Organized by the standards group that developed them...

http://www.w3.org/2002/ws/

• http://www.w3.org/TR/2004/NOTE-ws-arch-20040211/ - Web Services Architecture
• WsdlNotes - Web Services Description Language is an XML schema for documents that describe web services.

http://www.ws-i.org/

http://www.oasis-open.org/

• UddiNotes - "Universal Description, Discovery and Integration is the name of a group of web-based registries that expose information about a business or other entity[2] and its technical interfaces (or API's)."

Implementation

http://ws.apache.org/ - implementations of a few of the specs.

Misc

http://www.ibm.com/developerworks/webservices/library/ws-whichwsdl/ - good explanation of the differences between RPC and Document styles.