The Apache XML project is part of the Apache Software Foundation and focuses on XML-related projects.
It consists of several sub projects:

Projects related to webservices

SOAP: Is an old implementation of the SOAP. This project based on IBM's SOAP4J implementation. It should no longer used for new projects. Instead you should favour the Axis implementation.
XML-RPC: Apache XML-RPC is a Java implementation of XML-RPC, a protocol that uses XML over HTTP to implement remote procedure calls.
Axis: Apache Axis is the current implementation of the SOAP for Java and C++. It is the successor for the SOAP project.
WSIF: Web Services Invocation Framework is a simple Java API for invoking Web services.