It validates to make sure you own the game. You have to sit through and wait out the validation period. If they did not have this, it would be too easy for pirates to steal games and put Steam out of business.
Let it Validate
just get steam. it's fun and free anyway
If you hacked in steam games and you got caught by VAC and got permanent ban from their client you will have to make new steam account and repurchase all the games you have. From now on you can only play those games offline single player or custom games or you can play on none VAC secured servers if it will let you.
It needs a one-time internet connection to validate it, but otherwise, it does not need an internet connection to play. However If you want to play offline you must log into steam then go to offline mode. So you must have internet.
Just buy CS through steam and play.
This is possible, but only if the game already is available to buy from the Steam store. For example, if you have Half Life 1 from back when it was not on Steam, you can validate the CD key on Steam...and then play it without the disc through that. Games that Steam doesn't support however can not be translated over to Steam. Quite a few games are on Steam though, including EA recently adding a number of their games to it.
Steam automatically pauses downloads when you run a Steam game so the only option is to play a non-Steam game (something outside of Steam).
If You want to play on steam servers than yes, but if you want a non-steam version(You can only play Non-Steam Servers), you can download off megaupload, rapidshare, or torrents. Just google non-steam condition zero and the online patch, download them, and you got it!
Urm... you just buy Portal 2 from Steam and then play it...
You can't.
no it doesn't need steam
yes you have to have steam.