yes
Buy it through steam.
Technically, i do not believe it is supported by OS X. However, you can run windows on your mac via bootcamp, and play it that way. Not exactly the answer you were looking for, Im guiessing...
Steam will be available for the Mac sometime in April (See links below).
Its only on the PC.
Yes.On steam it is free for PC and MAC.
Civilization 5 is powered through the Steam system so you will need to install Steam and create an account to install and run the game. (See links below)
No. If you buy it on steam, then you will be able to download it to your steam games library and install it. After you do that, you'll be able to play it on steam. However, if you're on a mac, you can't play terraria. By the way, you have to buy the collector's edition to get a physical copy of the game.
Check steam tech support.
Yes you can.
No, because you are not using Steam to play the game, you are only using Steam to verify the game and to use the multiplayer parts of it. If you have a Mac version DVD/CD of the same game then you could still use it on steam if you register it on Steam.I THINK if you bought the Windows game on steam they will let you download the Mac version on the same machine. They have the full instructions on their web site.
A wide variety of games, available from the Mac's built in App Store, Steam and numerous other sources can be played on a Mac. Current popular games include Assassins Creed, Portal 2, Civilization V, Iron Marauders etc. etc.
You many be able to find it on the Steam community.