Hibernate is ORM tool and it provides the functionality of mapping Java objects to database. Hibernate tool actually interacts with database and performs database operations.
JPA is known as Java Persistence API and its an specification for implementing the persistence functionality. The implementation of JPA is known as persistence provider. Here are the implementation of JPA:
Hibernate is one of the JPA implementations. You can use Hibernate as one of the Persistence provider in your JPA applications.
More Hibernate Tutorials.