You should first ask the question to yourself and read the documentation, then, if you get other questions, ask them on the Hibernate forums (because your question is related to Hibernate and not to Seam).
Shortly: Java Persistence API (JPA) is a standardized API for persistence in Java. Hibernate is an implementation of JPA, as well as OpenJPA is another JPA implementation. Each implementation has specific features to extend the JPA functionalities. Seam supports any JPA implementation but is shipped by default with Hibernate.