A disk image is a copy of the entire contents of a storage device, such as a hard drive, CD-ROM, or DVD. It is typically created by software that can read the entire storage device and save it as a single file, which can then be used for data recovery. This is useful in data recovery because it allows the user to create a backup of the entire storage device, so that if the original device fails or is corrupted, the user can restore the data from the disk image.