Gatekeeper is a new system service available in Mac OS X 10.8 and later. Gatekeeper looks at downloaded programs to determine if they have been signed by a known developer identity and then either tells the user that the program was downloaded and makes sure they want to run it (if it was signed with a known developer ID) or tells the user that the program is from an unknown developer and does not provide the option to run it.
The only way a user of Mac OS X 10.8 will be able to run an App that is not signed, or that is signed by an unknown developer, is to turn the Gatekeeper off. Since it is unlikely that users will turn Gatekeeper off, if we want to share our App with others we need to sign it with a known Developer ID. Provided we are a member of the developer program, Apple will provide us with a Developer ID so that we can distribute trusted Apps outside of the App store.
Finally, if an App is modified (hacked or corrupted) after it has been signed, Gatekeeper will...