a number of things, the most important is that it makes an image of your whole hard drive or a part that you decide and then saves that image to a media of your choice(i.e. another HDD, CDROM, DVDROM, etc...) that way you will have a perfect copy of what you had so you can revert to it or put it on multiple media. For instance, I use it to back up my initial install of windows and all the drivers for my computer. So when i want a fresh install all i have to do is rewrite my drive back to the image i saved with Ghost instead of having to go through the entire install process again.