Perhaps the least intrusive way of notifying a user is with the status bar. This notification area allows us to display an icon, along with a scrolling 'ticker' style message which, if responded to with a long click, opens a 'sliding drawer' window that provides more message space and the opportunity for the user to generate some resultant action, usually the firing of an Intent and the starting of an Activity.
Both Activities and Services are able to produce status bar notifications. More often than not, we would use this from within a Service, that is, when our application is running in the background. Here for the sake of brevity we will call our notification from the main activity as the principles remain the same and this approach allows us to concentrate on how status bars work without having to worry about setting up extra components.