Hibernate allows you to persist objects to a relational database (ObjectRelationalMapper - ORM). From what it does it can be compared with Entity Beans. But it does it differently than EJB2 (the objects you perists are plain java objects, not heavy EJBs) and similar to EJB3 (in fact Hibernate 3 is the persistence mechanism in
JBoss Server)
Hibernate 3 implements the JPA (Java Persistence API).
best would be you go to hibernate.org and check out some of the tutorials. There is an upcomming book by Christian Bauer on Hibernate 3. Eventhough there are some books already out on hibernate 3 i would wait (you can get the Ebook already from Manning Early Access) for this one (or buy two books :-) )
pascal