Mountain Lion: About Gatekeeper
Some apps downloaded and installed from the Internet could adversely affect your Mac. Gatekeeper helps protect your Mac from such apps. Read this article to learn about Gatekeeper and its options.
Via: Apple Support
Personally, I think Gatekeeper is one of the better features of Mountain Lion, but it can be a cause of headaches if you haven’t brushed up on how it works. This article helps with that.