Every Android app will consist of components, a manifest file, and resources.

Components consist of four pieces:

Activities

• A single screen with a user interface (aka view)
• Most applications contain multiple activities
• When a new activity starts, it is pushed onto the back stack (Stack of Activities)
• User interface can be built with XML or in Java (Nice separation of UI code + Java logic)
• You can monitor lifespan of an activity through callback methods: onStart(), onPause(), etc.

Services

• They're what you use to perform long-running operations in the background
• Does not contain a user interface. THey're useful for network operations, background music
• Services run independently of the component that created it
• They can be bound to by other application components --if you allow them to.

Content Providers

• Used to store and retrieve data and make it accessible to all applications.
• This is the only way to share data across applications.
• It exposes a public URI that uniquely identifies its data set.
• Data is presented as simple as a simple table on a database model
• Android contains many content providers like contacts, e-mails, media

Broadcast receivers

• It responds to system-wide broadcast announcements
• Example, when the screen turns off, the battery is low, someone has taken a photo.
• Applications can initiate their broadcasts that others can listen to.
• Broadcast receivers contain no user interface.
• They can create status bar notifications to alert the user. "Notifications"

Android-Manifest.xml

• The purpose of this file is to give the Android system information about your application
• It describes the components in your applications
• It declares the permissions required to run the application
• Declares the minimum Android API level that the use requires