No.
You login via the Steam client, and the server tells the client what games are tied to your account. It then checks for the GCF (Global Cache Files) for each game to see what's "installed."
Then you download whatever you don't have and play it. The games are playable as long as a Steam account with said game is logged in.
It's kinda like XBL Marketplace items, only with the limitation that only the user who bought the item can use it (whereas on XBLM, I can download content and give it to a buddy for him to use, but if he deletes it he can't download it).
Bookmarks