Following are the most important interfaces of Hibernate which act as base for using Hibernate:

1. org.hibernate.SessionFactory

• SessionFactory includes all of the metadata about Object/Relational Mapping.
• SessionFactory is a thread-safe and immutable.
• The JPA equivalent of SessionFactory is EntityManagerFactory.
• A SessionFactory is very expensive to create. So an application should have only one SessionFactory for a given database.
• The SessionFactory maintains services that are used across all Session(s) such as connection pools, transaction system integrations, second level caches etc.
• The SessionFactory is immutable and its state can not be changed once created.

2. org.hibernate.Session

• Session represents a conversation between the application and the datastore.
• Session is single-threaded and its implementors are not required to be threadsafe.
• JPA equivalent of Session is EntityManager.
• The Hibernate Session wraps a JDBC java.sql.Connection behind the scenes.
• Hibernate Session acts a factory for org.hibernate.Transaction instances.
• The lifecycle of a Session is bounded by the beginning and end of a logical transaction.
• Hibernate Session provides create, read and delete operations for the entities mapped to the database tables.
• A Session instance is serializable if its persistent classes are serializable.
• If an exception is thrown in a Session, the transaction must be rolled back and the session is discarded.
• Hibernate Session maintains first level cache of the application domain model.

3. org.hibernate.Transaction

• Transaction is an abstraction of underlying transaction management.
• Transaction is associated with Session and is initiated by calling beginTransaction(). By calling commit() method, commits the current resource transaction, writing any unflushed changes to the database.
• Transaction is single-threaded and is used to define the physical transaction boundaries.
• The JPA equivalent of Transaction is EntityTransaction.
• Multiple transactions can be associated with a session or we can say that a session can span multiple transactions. But at a given time there should be only one uncommitted transaction associated with a session.

Other important interfaces

4. org.hibernate.query.Query

• Interface used to control query execution.
• Represents an HQL/JPQL query or a compiled Criteria query.
• Hibernate extension to the JPA Query/TypedQuery contract.

org.hibernate.Query is deprecated. Use org.hibernate.query.Query instead.

5. org.hibernate.Cache

• Provides an API for querying and managing the second level cache.
• Its methods do not adhere any isolation or transactional semantics associated with the underlying caches.

6. org.hibernate.Criteria

• Provides an API to retrieve entities.
• Criteria is simple to use when the number of conditions to be applied on the resultset are variable.