Hello 
!!
I am new in JPA / ObjectDB development and I have many questions:
1.1. If I understood correctly, ObjectDB is an implementation of JPA standard (currently 2.0)?
1.2. Is ObjectDB on the same level like Hibernate with only difference that Hibernate stores the data in a RDBMS and ObjectDB has its own format?
Cheers,
V.
Yes, ObjectDB supports JPA 2.0 (except several unimplemented features - see the issue tracking).
Hibernate supports JPA + its own old API + extensions. ObjectDB supports also JDO. But in the JPA context Hibernate and ObjectDB are indeed similar, where Hiberante uses an RDBMS for storage and ObjectDB uses its own internal object database engine, and therefore is usually much faster.
..is Objectdb somehow related to db4o?
No, except that db4o was also an object database, but ObjectDB was always much faster than db4o.