A disk image is a ?computer file containing the complete contents and structure of a ?data storage device. The term has been generalized to cover any such file, whether taken from an actual physical storage device or not.
An ordinary backup only backs up the files it can access; ?boot information, and files locked by an ?operating system or being changed at the time, may not be saved. A disk image contains all these, and faithfully replicates the data, so it is commonly used for backing up disks with operating systems, or bootable CD/DVD s.
The image is saved as a file, which for a full CD or system disk may be quite large (~10 MB to several GB). This file can be saved onto a ?hard drive, CD, DVD or other media, for later use.
Disk images can either be compressed using some type of ?compression algorithm like LZW, or uncompressed ("raw"). Images of CD-ROM s most often carry the file name extension .?iso, referring to the ISO 9660 file system commonly used on such discs. The .iso format is the most common format for Linux distributions]] and other online images. Other common CD image formats are .bin/.cue.
.bin files (or .RAW files) are images extracted in pure RAW format. That is 2352 bytes sectors, the full CD sector content: user data, sector header, error correction codes (ECC) and error detection codes (EDC). Once again, each sector is converted to digital data in the .BIN file, but more stuff is copied and the resulting file will be bigger. The .BIN file should be 251,000 x 2352 = 590,352,000 bytes big. This process will copy ANYTHING on the disc, so it is useful for exotic discs (multiple tracks, mixed track type Audio+Data or Data+Audio) and for non-PC CDs (PSX, VCD, MAC).
The ["iso image"] file is a digital copy of CD contents made this way: the ripper searches for the sectors of the CD that have been used, say 251,000 for instance (there are 330,000 sectors on a 74 min CD and 360,000 sectors on a 80 min CD). Each sector is copied on the .ISO file, one by one, and only 2048 bytes for each sector (only the ones containing the user data) are copied. The .ISO file should then be of size 251,000 x 2048 = 514,408,000 bytes. (It will be slightly bigger if the extractor puts a header on the file, like Nero .NRG files that are .ISO files plus a small file header).
.img is the filename extension usually use for the disk image of floppy disks, and sometimes, hard disks. First populated by DOS-based software HD-Copy ?DiskDupe and now ?WinRaWrite, is a handy way to archive a floppy disk completely, including bootable ones. In fact there is no "format" in it, just a raw dump of the content of the disk.
Some newer software like Some newer software like ?WinImage supports zipped version of the format and the extension is .imz.
A common use of disk images is for remote distribution of software such as Linux distributions: installation floppy disks or CD-ROMs can be recorded as disk image files, transferred over the Internet, and the contents of the original disk(s) duplicated exactly by end users with their own floppy or CD-R drives. So, user can burn the Isos to convert them in LiveCD s, to try an Operating System, without installing it in the hard disk .
Another common use is to provide virtual disk drive space to be used by emulators (i.e. ["qemu"]). This can prevent the CD from getting burned or damaged. It can also reduce bulk when one wishes to carry the contents of the CD along with oneself: one can store disk images to a relatively lightweight and bootable storage device which has a higher storage capacity than that of a CD (i.e. a USB keydrive).