You can backup individual drive letters (partitions) or entire disk drives.
A partition image includes all files and folders independent of their attributes (including hidden and system files), boot record, FAT (file allocation table), root and the zero track of the hard disk with master boot record (MBR).
A disk image includes images of all disk partitions as well as the zero track with master boot record (MBR). To ensure recovery from complete disk failure, you should backup entire disk drives. Only by backing up entire disks will you capture hidden recovery partitions that may have been installed by your PC system vendor.