Gatekeeper lets Mountain Lion users control where their apps come from. They can choose to allow app downloads from anywhere — just as they have always been able to do — or limit downloads to the Mac App Store and trusted developers. By adding their Developer ID to apps, developers can “sign” their apps to be trusted.
The downside of digitally signing apps is that Apple will have the ability to shut down apps that they deem dangerous.
The update also marks a change in how they get Xcode since it now downloads as an app directly from the Mac App Store instead of as an installer.
Xcode 4.3 is free and available for download now.