The interface is pretty nice and simple and it’s very easy to use and navigate as well, just like you know it from the iOS App Store. Purchases are made using your iTunes Store account, also just like the iOS App Store.
To get Mac App Store on your Mac, you’ll need to fire up Software Update and install the new Mac OS X 10.6.6 update which doesn’t have much else to offer than the App Store. After installation it will appear in your Dock as a blue app icon.
Learn more about the Mac App Store at Apple.