1) It is a tool used by developers to display 3D graphics
2) no, you must move to Windows 7 to get 10 or higher
3) I don't have a clue (haven't kept up with it). You need newer hardware to use 10/11. Since you have so many questions you probably wouldn't even notice the difference.
4) no, if they run crappy on 9, they'll run crappy on 10/11
5) no, XP should be fine. Depending on your PC you might be able to upgrade a few components
6) no, the average user shouldn't have to know all the technical details. It is one of the reasons why home consoles are so great.
EDIT: I assumed the game you were running is really a DirectX9 game, not a DirectX 10/11 game. If a game is DirectX10 compatible and you have DirectX10 compatible hardware, then it would might make sense to upgrade windows, but this really depends on the game and what it uses in DX10.
