Your games are locked into steam, and they may never be removed from there. You own none of them. The only way to play them is to open Steam (presuming it's up / not horribly busy / you have an internet connection^) and be shown their market place. There is no way to 'export' your 'purchases'.
Contrast that with other sites where you buy the game and get to download an actual installer, which you can do whatever you like with.
Steam could do a lot better for users, but by locking people into their service they do a lot better for themselves.
^There is an offline mode, yes. Except you have to put it into offline mode while you still have an internet connection, which sucks (what if your internet goes down due to a power cut / accident / misconfiguration at the ISPs end?), and it's not supported by all games.