init.ora - Usually located in $ORACLE_HOME/dbs. In some unix installations, this entry is a link to $ORACLE_BASE/admin/$ORACLE_SID/pfile. The init.ora file contains parameters that specify memory, database defaults, etc. that the database uses at startup.

control files - These files are specified in the init.ora file by the CONTROL_FILES parameter. There are usually multiple copies of the control file on multiple disks. The control files are binary files that keep track of the files that compose the database and which SCN (transaction) is current.

alert.log - This file location is specified in the init.ora file by the backround_dump_dest parameter. This file is usually in $ORACLE_BASE/admin/$ORACLE_SID/bdump. The alert.log file is a log of database level events that should be monitored by a human.