http://docs.oracle.com/cd/B19306_01/server.102/b14200/statements_5003.htm

RESETLOGS  Specify RESETLOGS if you want Oracle Database to ignore the contents of the files listed in the LOGFILE clause. These files do not have to exist. Each redo_log_file_spec in the LOGFILE clause must specify the SIZE parameter. The database assigns all online redo log file groups to thread 1 and enables this thread for public use by any instance. After using this clause, you must open the database using the RESETLOGS clause of the ALTER DATABASE statement.

NORESETLOGS  Specify NORESETLOGS if you want Oracle Database to use all files in the LOGFILE clause as they were when the database was last open. These files must exist and must be the current online redo log files rather than restored backups. The database reassigns the redo log file groups to the threads to which they were previously assigned and reenables the threads as they were previously enabled.

区别在于是否还需要当前的在线日志文件组