You cannot ignore Windows Libraries if you wish to use File History
“File Explorer” is the file manager application in Windows that lets you see what files and folders you have on your computer and where they are. You can then move, copy, delete, open programs, open data files, and so on from within a File Explorer window. Note that this is the same function that was previously called “Windows Explorer”.
With the advent of Windows 7 a new concept was introduced into File Explorer. This was the concept of “Libraries”. They appear in the navigation pane at the lefthand side of a File Explorer window. I confess that I’ve often avoided defining “libraries” when introducing my computer support clients to File Explorer as they can be very confusing until you realise what they are. I would probably still be avoiding the issue except that the concept of Libraries is central to how Windows 8’s backup feature works (as mentioned in my blog on Windows 8 File History).
So, what are Microsoft’s Libraries? Maybe my mind is a bit too literal (pedantic even?), but when I think of a “library” I think of a physical collection of “things” such as books, magazines, CDs etc. The main point about a library (to this pedant, anyway) is that all of these things comprising the library are all to be found in one place. A library is a physical thing that actually includes its contents!

These are the default libraries in Windows 7 and 8
So, for instance, you could have a library that contains all of your folders that relate to your clients. You might have spreadsheets relating to clients and word processing documents relating to clients but these could easily be in different parts of your drive (if you tend to keep all your spreadsheets together and all your word processing documents together). It would be quite simple and sensible to create a “Clients” library and to include client spreadsheets and client word processing documents in that library.

A proper library – this is the new one in Clapham
Now, you might think I’m making a bit of a meal of explaining a very simple concept. If so, I apologise, but my experience is that for every person that grasps the concept easily, there are many more that can’t get their heads around it.
To my mind, the whole thing would have been a lot simpler to understand if Microsoft had just been a little bit more prosaic and literal in their nomenclature. Why didn’t they just call them “lists” instead of “libraries”? Everyone knows what a list is.

A user-defined library (“Items Backed Up”) has been added here. Note that folders can be included in more than one library (the folder “Documents” appears in two libraries here)
As I’ve already said, I’ve tended to avoid trying to get that across to people just learning about files and folders and so on, but you will find Windows 8 File History very limiting if you don’t get to grips with it as it deals in libraries as its “unit” of stuff to back up. It is libraries that it backs up, and any files and folders that have not been added to a library will not be backed up. Also, any libraries that you have created (over and above the pre-existing default libraries) will not be backed up unless you add them to File History’s schedule.