JAXB is one of the API’s in the Java Web Serviced Developer Pack.

From the Sun web site:

JavaTM Architecture for XML Binding (JAXB) provides an API and tools that automate the mapping between XML documents and Java objects.
JAXB makes XML easy to use by compiling an XML schema into one or more Java technology classes. The combination of the schema derived classes and the binding framework enable one to perform the following operations on an XML document:

• Unmarshal XML content into a Java representation
• Access, update and validate the Java representation against schema constraint
• Marshal the Java representation of the XML content into XML content
  

JAXB gives Java developers an efficient and standard way of mapping between XML and Java code. Newer features include:

• Support for a subset of W3C XML Schema and XML Namespaces
• More flexible unmarshalling and marshalling functionality
• Validation process enhancements

The JAXB API is here.

JAXB is compliant with JSR-031.