1) DL and install Steam.
2) DL and install any of the games you've purchased (they're all listed under the "My Games" tab as either
Installed or
Not Installed - clever, huh?

).
3) Uninstall one if you want/need to (or play from a different computer) - you can reinstall any game that's listed in your "My Games" list. I have yet to reach a limit on the number of times but I've installed CS only 5 or 6 times.
4) Once games are installed, you can start them via a shortcut. I think Steam starts up but only for a short time, just enough to validate your installation. I believe there is a way to play the come without Steam starting up at all (except for games like CS, DoD, TF2) but I haven't tried it out.