anything bought under your steam account can run on your steam account no matter what machine your on (though it will have to re-download the game files on the local machine), all you have to do is install Steam and use your same steam account.
Anything with the Steam Play logo with the Windows and Mac logos on it means you get both versions... so if you install and run steam on Windows you can play it, or install Steam on Mac and play it as well without having to re-buy a separate Mac version.
Steam is great for gaming... you don;t have to keep rebuying software for different platforms (usually), and you don't have to mess with discs or having them in your drive or any junk like that. Just make sure you've set steam up for offline mode right for a game if your going to play without an internet connection. it does copy protection with the steam servers, so many games by default won't run if you don't have a connection... but there is an offline mode for that.