From the community documentation:

hibernate.hbm2ddl.auto Automatically validates or exports schema DDL to the database when the SessionFactory is created. With create-drop, the database schema will be dropped when the SessionFactory is closed explicitly.

e.g. validate | update | create | create-drop

So the list of possible options are,

  • validate : validate the schema, makes no changes to the database.
  • update : update the schema.
  • create : creates the schema, destroying previous data.
  • create-drop : drop the schema when the SessionFactory is closed explicitly, typically when the application is stopped.
  • none : does nothing with the schema, makes no changes to the database

These options seem intended to be developers tools and not to facilitate any production level databases, you may want to have a look at the following question; Hibernate: hbm2ddl.auto=update in production?