One of the important changes that OS X Mountain Lion brings them has to do with the system Gatekeeper. In practice, Apple has implemented a security system in OS X that allows us to install only applications from the Mac App Store or applications from developers verified by Apple, but of course we also have the option to install anything, from anywhere. The problem is that this Gatekeeper is natively activated in OS X Mountain Lion and only when you try to install an application from an unverified developer will you be warned by the system that you are not allowed to do this.
Through this system, Apple tries to control OS X in a way similar to the one in which it controls iOS, but for now we have the option to install whatever we want. To disable the Gatekeeper or to modify its settings, you must go to System Preferences>Security, click on the lock icon on the left side and in the category Allow applications downloaded from to choose Anywhere to install applications from any source you want. I suppose that in a few years we will be forced to install only applications from the Mac App Store, but for now we still have the freedom to choose.
