Android Application Component - Activity, Service, Broadcast Receiver, Content Provider

Application Components are the essential factors of application. Each can be entrance to the application. Each must be register in AndroidManifest.xml file with short detail like intent-filter etc. and interaction of them.

Activity

It represents, a single screen of user interface with performing action. In Simple word, we can say that activity is a User Interface with back-end class to handle the action performed on User Interface. 

Service

It is the component of application running in background for each reason. It consists of a back-end class with no User Interface.

Broadcast Receiver

Android Operating System broadcast message/intent to all application installed on the application. This messages are those messages, which is no time bound and may may occurs anytime. e.g Incoming Call, Device Boot. So system broadcast it to all application and those application will capture it with Broadcast Receiver to perform an action based on it. Broadcast Message are events, that is out from normal user flow. It respond to the Broadcast Messages of system. It Consists of Back-end class with no User Interface. 

Content Provider

It is a shared set application data, stored on a steady and persistent storage like file, web, Database etc. Other application can query the application on request.

Other Sub Components

AndroidManifest

It is a configuration file of application, having an application summary.
Resources

Consists of used resources in application like Strings, Colors, Style, Images

Assets

Consists of used resources in application other than Resources components like Fonts, audio, video etc.
Layout

Views arrangement to build a screen or front-end design.
View

Elements of user interface, that are used to design a layout or screen. Like Button, TextView, Layout etc.
Intent

It's asynchronous message between application components or with third party application.
Fragment

A portion of user interface of an Activity.