In Mountain Lion, OS X software updates are available via the App Store app. System software updates when available are displayed in the same “Updates” tab of the App Store app. App Store app is now the central location for updates to system software and apps.
The “Software Update…” on the Mac menu will launch the App Store app, instead of launching a separate window as in OS X Lion. App Store app will notify via Notification Center when updates are available for OS X or purchased apps.
By default, Mountain Lion will automatically download OS X software updates and app updates so they’re ready to be installed. This setting is available from System Preferences > Software Update.
As shown in the preference pane, you can enable Mountain Lion to automatically download apps purchased on another Mac. This is similar to app push feature available in iOS 5.