Hi,
Did you look at the examples in Hibernate docs for one-to-many relationships? Also look in the JEE 5 tutorial chapter 24: introduction to JPA
http://java.sun.com/javaee/5/docs/tutorial/doc/
A few things you must always answer before deciding the nature of a relationship:
1. how will a user create a new A object? will it first be created without B's and then later you will add them? will A be created when it already has some B's associated to it? maybe both options are needed?
2. what happens when A is removed? will all its B's also be removed?
3. will you allow 2 users to concurrently update 2 different B's that "belong" to the same A object?
4. how are the various classes derived from your abstract class mostly expected to differ? in their logic? in their data members?
5. Will your database also be accessible from non-JPA code?