JDO, JPA and Hibernate

1. JDO and JPA are specifications. They specify an interface to abstract the database technologies.

2. Hibernate is an implementation of JPA.

3. DataNucleus is an implementation of both JPA and JDO.

Java Data Objects (JDO) is a standard interface for storing objects containing data into a database. The standard defines interfaces for annotating Java objects, retrieving objects with queries, and interacting with a database using transactions. An application that uses the JDO interface can work with different kinds of databases without using any database-specific code, including relational databases, hierarchical databases, and object databases. As with other interface standards, JDO simplifies porting your application between different storage solutions.


Java Persistence API (JPA) is a standard interface for storing objects containing data into a relational database. The standard defines interfaces for annotating Java objects, retrieving objects using queries, and interacting with a database using transactions. An application that uses the JPA interface can work with different databases without using any vendor-specific database code. JPA simplifies porting your application between different database vendors.


Reference:
http://code.google.com/intl/eclipse/appengine/docs/java/datastore/jdo/overview.html