Every Oracle database has a control file.
Every time an instance of an Oracle database is started, its control file identifies the physical structure:
that must be opened for database operation to proceed.
A control file contains also entries that specify:
If the physical makeup of the database is altered (for example, if a new datafile or redo log file is created), then the control file is automatically modified by Oracle Database to reflect the change. A control file is also used in database recovery.
The parameter file has a listing of all the control files and the control file is responsible for storing information about all the other data files and resources used by the database.
If the control file can't be read the database doesn't know what else exists! This is why you should be careful to keep good backups of your control files using plain of system backups. This is also why Oracle maintains multiple copies (typically 3) of the control file for safety. It multiplex the control file, that is, simultaneously maintain a number of identical control file copies, to protect against a failure involving the control file.
select name from v$controlfile;