for example i have 2 steam accounts i log in to the old one, does that mean it got downloaded into the computer? or is it just a log in? ..

and i mean the steam account .. does it download into the hard drive? ..

the account is not on your HDD, but you can still play without being Online by utilizing "Offline mode"

oh ok .. so it doesn't download? ..

its an Account, no Program Downloads an Account, it simply remembers the Account it uses that data to run the Software.

Not at all. I know this because my work has its computers rigged not to allow any downloads at all, and I can still fo to the Steam website and log in.

I haven't done it in a year though. I have no clue if anything has changed.