Image restoration refers to the recovery of an original Image that has got dust, scratches, lines and blurriness with the time.
For sure, you must have a couple of pictures that are very important and precious to you but they are too old to look good. They have stains and scratches. Image restoration is just like reincarnation to those pictures.
It is different from image enhancement. Image enhancement is to emphasize features of the image that make the image more pleasing to the observer, while restoration is a process where experts try to restore to the point when it was clicked and how it looked the very first time. Though results slightly differ but make a justified difference.