Database backups created by Oracle Recovery Manager (RMAN) are stored as image copies or backup sets.
Image copies are exact byte-for-byte copies of files. You can create an image copy by copying a file at the operating system level. Unlike copying files at the operating system level, however, image copies created through RMAN are recorded in the RMAN repository so that RMAN can use these copies during database restore operations and recovery. RMAN can restore files only if they are recorded in the RMAN repository. RMAN can create image copies only on disk.
Backup sets are logical entities produced by the RMAN BACKUP command. This command can produce one or more backup sets on disk or tape devices. Although image copies cannot use all RMAN features, their advantages are that you can apply incremental backups to them (synthetic full backups) and you can use them directly in place without first copying them, for very fast restores.
Each backup set contains one or more physical files called backup pieces. A backup piece stores the backup of one or more database files in a compact RMAN-specific format. One advantage of backup sets is that RMAN uses unused block compression to save space in backing up data files. Only those blocks in the data files that have been used to store data are included in the backup set. Backup sets can also be compressed, encrypted, sent to tape, and use advanced unused-space compression that is not available with datafile copies.