Steam is register your singleplayer game online and then play it offline DRM.
I disagree. As far as I understood Steam, it's meant to to be so that most games just cannot be played without Steam which must always be on - except for a few cases of "offline mode". I once tried it with "Team Fortress" for which I once got a code on a Games Com - it didn't want to play offline. It threw me out. Even after using Steam's "Offline Mode". I think it even accused me of hacking, because I didn't connect to the servers.