Java API for XML-Based RPC (JAX-RPC) is a Legacy Web Services Java API, it uses SOAP and HTTP to do RPCs over the network and enables the building of Web services and Web applications based on the SOAP 1.1 specification, Java SE 1.4 or lower.JAX-WS 2.0 is the successor to JAX-RPC 1.1. JAX-WS still supports SOAP 1.1 over HTTP 1.1, so interoperability will not be affected. However, there are lots of differences:
JAX-WS maps to Java 5.0 and relies on many of the features new in Java 5.0 like Web Service annotations.
JAX-RPC has its own data mapping model, JAX-WS's data mapping model is JAXB. JAXB promises mappings for all XML schemas.
JAX-WS introduces message-oriented functionality, dynamic asynchronous functionality which is missing in JAX-RPC.
JAX-WS also add support, via JAXB, for MTOM, the new attachment specification.