Not trying to spark any arguments, but as there are some steam experts around: I have read about steam backup. Does that work in the same way as a GOG that is saved onto an external hard drive. Or will the games always require online and steam?
Yes, it takes all the game data and saves it to an external drive, but reinstallation from the backup requires being online. It's basically the same as a regular installation through Steam, only instead of downloading the data, the client loads it from the local backup (and then proceeds to patch the game to the latest version).
EDIT: Also, contrary to what spinefarm says, the function is not there for when you need to reinstall Windows. If you ever need to do that, all you have to do is take the steamapps and userdata folders as they are and back those up. Then when you reinstall Steam, you just drag them back.