A broadcast receiver is a component that does nothing but receive and react to broadcast announcements. Many broadcasts originate in system code — for example, announcements that the timezone has changed, that the battery is low, that a picture has been taken, or that the user changed a language preference. Applications can also initiate broadcasts — for example, to let other applications know that some data has been downloaded to the device and is available for them to use.
An application can have any number of broadcast receivers to respond to any announcements it considers important. All receivers extend the BroadcastReceiver base class.
Broadcast receivers do not display a user interface. However, they may start an activity in response to the information they receive, or they may use the NotificationManager to alert the user. Notifications can get the user's attention in various ways — flashing the backlight, vibrating the device, playing a sound, and so on. They typically place a persistent icon in the status bar, which users can open to get the message.