You do not need steam to be online to use your games .
Many options to backup,the most obvious one is using the built in steam backup/restore utility.
Or you can just copy paste the individual game folders or the whole of the steamapps/common folder to usb etc.
I personall have a complete backup of my entire games folder on usb drive.
The idea bove regarding torrenting is the worst possible thing you could do.
Please be aware that even if you legally own a copy of your game you CAN NOT legally torrent a copy for backup due to the fact that when you download you upload as well.
If you upload then you can be held responisble for illegal file sharing,i suggest the guy who mentioned that above doesnt advise that again.
Even if you did torrent them and burnt to a CD im not actually sure that steams folders and updates are compatible with retail CD versions,i think there is a little difference with DIgital downloads,correct me if im wrong.
Basically you can just drag the content where you want and then when reinstalling into steam the steam cleint will check all files are complete and redownload any missing/corrupt files.
Also serials for all steam games can be found in steam client on games page to the right.