JAX-RCP vs JAX-WS

JAX-RPC
A specification/API for Java developers to develop SOAP based interoperable web services. This API is now obsolete, and may be dropped from the next JEE version.

JAX-WS
The successor to JAX-RPC. It requires Java 5.0, and is not backwards-compatible to JAX-RPC. This article describes the high-level differences to JAX-RPC.

SAAJ
Another specification/API for using SOAP envelopes with or without attachments. It operates on a lower level than JAX-RPC or JAX-WS, both of which will use SOAP envelopes based on SAAJ if needed.

JAX-RPC 2.0 (Java API for XML Based RPC) has been renamed to JAX-WS 2.0 (Java API for XML Web Services).

This was done for a number of reasons, but the main reasons are:
The JAX-RPC name is misleading, developers assume that all JAX-RPC is about is just RPC, not Web Services.

By renaming JAX-RPC to JAX-WS we can eliminate this confusion. JAX-RPC 2.0 uses JAXB for all databinding.

As the bindings are different in JAX-RPC 1.1, it introduced a number of source compatibility issues. The migration from JAX-RPC 1.1 to JAX-RPC 2.0 is not cookie cutter as generated Java code and schemas will be different than those generated by JAX-RPC 1.1.

Although the renaming does not ease this migration, it does let the developer know that these are two separate technologies, hence the more difficult migration is more palatable.

Maintaining binary compatibility with the JAX-RPC 1.1 APIs, was hindering our goal of ease-of-development. Because we had this binary compatibility requirement, many legacy APIs were exposed such as the various methods on javax.xml.rpc.Service and the javax.xml.rpc.Call. Having these legacy APIs around would confuse developers. By renaming JAX-RPC 2.0 to JAX-WS 2.0 we no longer have this binary compatibility requirement and we can rid the APIs of these legacy methods and interfaces and we can focus on a new set of APIs that are easier to understand and use.

WebService Implementations
Apache Axis
An open source implementation of the Java WS APIs for sending and receiving SOAP messages. Axis 1 supports JAX-RPC and SAAJ, while Axis 2 supports SAAJ and JAX-WS.

Apache SOAP
The first SOAP implementation. It is now obsolete. It's better to use Apache Axis to avail oneself of the latest features.

Sun JWSDP
Sun Java Webservices Developer Pack, is an implementation of JAX-RPC, SAAJ and various other XML Java technologies. It is now deprecated in favor of GlassFish.

Metro
The web services stack used in GlassFish?. It supports SAAJ, JAX-WS, WS-Security and other standards.

JAX-RS
It is a Java API for RESTful web services.
Jersey is the reference implementation of the JAX-RS API, as defined in the JSR-311 standard for RESTful web services.

For more info, see http://faq.javaranch.com/java/WebServicesFaq.