JourneymanGeek's answer is good for the technical details, see the linked pages here and here. I've tried to approach this from the end result angle.
Will games using these features work on windows 7/8?
Regarding new games designed for windows 8 working on windows 7, see the below article:
and the relevent quote:
DX11.1 will be in Windows 8 and Microsoft will make a download available to Windows 7 users.
I do not know of the orignal source of that information, however it seems legit.
What we can take from this is that even desktop games/apps that take advantage of new features in DX11.1 will still work providing you download the update to DX on windows 7.
Regarding old games designed for earlier versions of windows working on windows 8, see below:
It appears there are some breaking changes, I've found several reports of older games not working, the site below seeming fairly objective (forgive the colour scheme):
--Dead link removed--
Further info: --Dead link removed--
According to this question: Does Steam Work On Windows 8? steam does work, although not yet officially supported.