Image copies: backup like 'cp' command in normal ----- this is one type of RMAN bkp format
backup set : collection of pieces --- this is one type of rman bkp format
Backup Pieces : backup of some datafiles.
Backup Set :
1. By Default RMAN backups are in Backup Set Formats.
2. Each Backup Set contains one or more backup pieces.
3. Backup Set contain One or More datafiles, Archived redo log files or Controlfiles.
Image Copy :
1. RMAN image copy and a copy you make with an operating system copy command such as dd (which makes image copies) are identical.
RMAN can write blocks from many files into the same backup set. but you can’t do so in the case of an image copy—an image copy is identical, byte by byte, to the original datafile,control file, or archived redo log file.