Backing up, is just making a copy of your personal files (photos, documents, etc.)
, and saving it in another location.
It's a good idea to backup your personal files, on a regular basis.
I do it once a week, but you can do it as often as you feel is necessary.
I use Cobian, to back up my personal files.
Once it's set up, back ups are just a matter of clicking a button.
If you want to back up system files, you would need a disk imaging program, like Acronis True Image
With that, you can image (copy)
your operating system, and program files, to an external drive.
That way, if your harddrive ever crashes, you can copy the image to the new drive, which should restore all of you Operating System files, and installed program files.
Acronis can also be used to back up your personal files.
In addition to backing up your personal information to an external drive, it's a good idea to back it up to CD/DVD, also.
That way, if your external drive also dies, you won't lose your personal files.