Important Android component's

An Android application consists out of the following parts:

• Activity - Represents the presentation layer of an Android application, e.g. a screen which the user sees. An Android application can have several activities and it can be switched between them during runtime of the application.
  
• Views - The User interface of an Activities is build with widgets classes which inherent from "android.view.View". The layout of the views is managed by "android.view.ViewGroups".
  
• Services - perform background tasks without providing an UI. They can notify the user via the notification framework in Android.
  
• Content Provider - provides data to applications, via a content provider your application can share data with other applications. Android contains a SQLite DB which can serve as data provider
  
• Intents are asynchronous messages which allow the application to request functionality from other services or activities. An application can call directly a service or activity (explicit intent) or ask the Android system for registered services and applications for an intent (implicit intents). For example the application could ask via an intent for a contact application. Application register themself to an intent via an IntentFilter. Intents are a powerful concept as they allow to create loosely coupled applications.
  
• Broadcast Receiver - receives system messages and implicit intents, can be used to react to changed conditions in the system. An application can register as a broadcast receiver for certain events and can be started if such an event occurs.
  

Other Android parts are Android Widgets or Live Folders and Live Wallpapers . Live Folders display any source of data on the homescreen without launching the corresponding application.

Security and permissions
Android defines certain permissions for certain tasks. For example if the application want to access the Internet it must define in its configuration file that it would like to use the related permission. During the installation of an Android application the user get a screen in which he needs to confirm the required permissions of the application.