A ROLLBACK reverses all changes that have been implemented by a transaction or a subtransaction in the database instance. The database system then releases all locks that have been allocated to the transaction, implicitly deletes all results tables that have been generated in the transaction, and opens a new transaction.
Any changes that are have been completed by a COMMIT can no longer be reversed by a ROLLBACK.
In normal database operation, the database system performs the necessary ROLLBACK actions itself. However, a ROLLBACK can also be requested explicitly using the appropriate SQL statements.
See also:
Reference Manual, ROLLBACK Statement