How does the mail icon count number show up on the iPad / iPhone

I want to use the same number count like the mail icon for th开发者_开发百科e iPhone and iPad how does that small number pop up over an icon?

[UIApplication sharedApplication].applicationIconBadgeNumber = 3;

Remember to set this to 0 when you want the badge to go away, so it doesn't get stuck there annoyingly.