Alarms in Android are just like normal alarms we use to remind us about events or that wakes us up in the morning. They serve to tell your app that the time has come to do something. What you’re going to do is up to you and you can use the little note you left on the alarm to distinguish between different events.
So you have your app, and you want to fire up that beautiful notification to remind the user that he hasn’t been using it for some time now and you wander how to do it. Well you may go and read the official docs over at Android Developers and wonder what the heck, or just scroll down and see the gists.