Types of Database Errors 
There are a number of errors that affect the operations of an Oracle database or can cause the database to crash. Depending on the type of error, a recovery can either be performed automatically or must be performed by the user or database administrator.
Therefore, you must find out the exact type of error so that you take the appropriate action to correct the error.
Errors can generally be divided into the following categories:
User errors, see Recovery after User Errors
Statement errors, see Recovery after Statement Errors
Process errors, see Recovery after Process Errors
Instance errors, see Recovery after an Instance Error
Media errors, see Recovery after Media Errors