Four types of files are used by the Oracle instance, and comprise the database:
These files (usually called the INIT.ORA files) specify how the instance is configured and point to the control files.
These files contain the names of all the datafiles and the online and archived log files.
These files contain a record of all changes made to the database.
When you START the instance, the parameter file is read. When you MOUNT the database, the control files are read. When you OPEN the database, the datafiles are referenced. When a database is being updated, changes are recorded in the online log files, which provide a mechanism for recovery in case of a failure.