Apache Axis is an implementation of the SOAP ("Simple Object Access Protocol") submission to W3C.
From the draft W3C specification:
SOAP is a lightweight protocol for exchanging structured information in a decentralized, distributed environment. It is an XML based protocol that consists of three parts: an envelope that defines a framework for describing what is in a message and how to process it, a set of encoding rules for expressing instances of application-defined datatypes, and a convention for representing remote procedure calls and responses.
This project is a follow-on to the Apache SOAP project.
Please see the Reference Library for a list of technical resources that should prove useful.