When you back up files and folders, only the files and folder tree are compressed and stored.
Disk/partition backups are different from file and folder backups. Acronis True Image 2013 stores a sector-by-sector snapshot of the disk or partition. This includes the operating system, registry, drivers, software applications, data files, and system areas hidden from the user. This procedure is called "creating a disk image," and the resulting backup is often called a disk/partition image.
By default Acronis True Image 2013 stores only the hard disk sectors that contain data.
Furthermore, Acronis True Image 2013 does not back up pagefile.sys under Windows XP and later and hiberfil.sys (a file that keeps RAM contents when the computer goes into hibernation). This reduces image size and speeds up image creation and recovery.
A partition image includes all files and folders (including hidden and system files), boot record, and FAT (file allocation table). It also includes files in the root directory and the zero track of the hard disk with the master boot record (MBR).
A disk image includes images of all disk partitions and the zero track with the MBR.