When you reimage a hard disk, you restore the entire disk from a disk image file. Since this restore process involves erasing all the current data on the hard disk, it is typically used a last resort system recovery option.
The disk image used to restore a computer may be a recent backup from the computer's hard drive or it may be disk image with completely new data. Most home users would reimage a hard disk from a personal backup file, such as a Norton Ghost disk image (Windows) or a Time Machine backup (Mac). Reimaging from a backup file allows you to recover all the data that was saved at the time of the backup. System administrators, on the other hand, may reimage business computers using standard disk images instead of backup files. This ensures all computers within a department have the same data.
Reimaging is a useful system restore method, since it rebuilds a hard drive with the exact content saved in the disk image. This includes all the folders and files as well as the hard disk file system information. Therefore, reimaging is one-step process, which doesn't require multiple installations. While the term "reimage" is sometimes used synonymously with an OS reload (or operating system reinstallation), it technically refers only to the process of restoring data from a disk image.
Reimage is also the name of a Windows PC hard disk repair utility.