A disk image is a snapshot of the complete disk, containing all information, including some of which you might not be aware:
- Boot partition: how the computer first starts
- Active operating system partitions: usually, at east one partition for each OS, such as Windows and NTFS
- Hidden Windows Restore partition: contains information to return to original state.
Above is an example of all that would be saved in a disk image of a particular PC.
An image is an excellent safety-net, allowing you to restore everything, or to open the image in a browser and restore only what is needed.
A Windows System Image “is an exact copy of all system disks which can be used to restore your PC to the state it was in at the time the image was made. By default, a system image only includes the drives that Windows requires to run, including Windows 10 itself, your system settings, programs, and files.” This would not include other partitions, should you make the system dual boot.
My personal experience is that it is far faster, and far safer, to rely on a disk image made with a reputable tool such as Macrium Reflect Free or an alternative. These are dedicated to making, to displaying, to restoring and to verifying images, since they show just what is being retained. It surprises that it is faster to restore the entire C: drive, when needed, than to use Windows Restore.
If you make images on a regular basis on separate media, e.g. a removable disk drive, you have backup even if your HDD or SSD dies.