[Coda]

It's an emergency service set up through the cell system. It's capable of sending an alert to all cell phones in the area.

If you want to disable the alerts (because they're loud, terrifying, unexpected, and usually irrelevant because they broadcast them to WAY bigger of an area than is actually relevant):

iPhone:
* Launch the Settings app on your iPhone.
* Tap on Notification Center and scroll all the way to the bottom.
* Under the Government Alerts section, toggle the AMBER Alerts or Government Alerts option on or off to enable or disable them.

Android: See http://www.androidcentral.com/amber-...-you-need-know because it varies from version to version and phone to phone.