So before asking this here I did some research and I found some answers but I have no idea what's right. The official page of Steam box says:
You can play all your Windows and Mac games on your SteamOS machine, too.
But then I read posts of some people for example:
Q.: What games run on SteamOS?
A.: Any Steam game with Linux support should run on SteamOS, although Valve will pin down this list as the launch date approaches. There are roughly 300 Linux games available through Steam, including major titles like "Europa Universalis IV" and indie darlings like "Fez."
And some forum posts about not buying it because you can't play windows games.
So I am confused now. Can you or can you not play windows games on the Steambox / Steam OS?