Perhaps you tried to open some application or mount some DMG on your Mac and encountered the following alarming message
"[Application] is damaged and can't be opened. You should move it to the trash."
Perhaps it is indeed damaged. But more likely it is just not signed by its developer or not made available from the AppStore, depending on how tight your security preferences are set. A less misleading message would certainly be nice.
If it's not really damaged then how do you get the application to open?
If you trust the people from whom you are downloading things you can bypass the warning by adjusting your 'Preferences > Security & Privacy' to
Allow applications downloaded from: Anywhere
It is sufficient to do this once when you first open the application, and it's probably best to re-tighten the preferences once it is successfully launched.
So who is it that does not sign their applications and causes you all this trouble?
Me, for one. None of my software is signed and none of it is distributed via the AppStore. The ability to sign software depends on paying one hundred dollars a year to Apple become a 'registered developer'. (I understand that similar plans are in the works for Microsoft developers). As a result, many of us open source software developers have not signed our applications.
Now, as it happens I have just ponied up so as soon as I figure out how to get my code into XCode and sign it you won't get the warning from my software. But before then, if you're wondering whether you should trust me enough to put your shields down, you can always ponder the source code, or be reassured that others can.
So now you know what the alarming message probably actually means, and also what you can do about it.