Android operating system is a stack of software components which is roughly divides into five sections and four main layers.
Linux Kernel
This provides a level of abstraction between the device hardware. It contains all the essential hardware drivers like camera, keypad, display etc. Also, the kernel handles all the things that Linux is really good at networking and a vast array of device drivers. Which take the pain out of interfacing to peripheral hardware.
Liberaries
On top of Linux kernel there is a set of libraries including open-source Web browser engine. WebKit, well known library libc. SQLite database which is a useful repository for storage and sharing of application data, libraries to play and record audio and video, SSL libraries.
Android Libraries
This category encompasses those Java-based libraries that are specific to Android development. Examples of libraries in this category include the application framework libraries. In addition to those that facilitate user interface building, graphics drawing and database access. A summary of some key core libraries available to the developer is as follows −
- android.app − Provides access to the application model and is the cornerstone of all applications.
- android.content − Facilitates content access, publishing and messaging between applications and application components.
- android.database − Used to access data published by content providers and includes SQLite database management classes.
- android.opengl − A Java interface to the OpenGL ES 3D graphics rendering API.
- android.os − Provides applications with access to standard operating system services including messages, system services and inter-process communication.
- android.text − Used to render and manipulate text on a device display.
- android.view − The fundamental building blocks of application user interfaces.
- widget − A rich collection of pre-built user interface components. Such as buttons, labels, list views, layout managers, radio buttons etc.
- webkit − A set of classes intend to allow web-browsing capabilities to be built into applications.
Application Framework
The Application Framework layer provides many higher-level services to applications in the form of Java classes. Application developers are allow to make use of these services in their applications.
You will find all the application at the top layer. You will write your application to be install on this layer only. Examples of such applications are Contacts Books, Browser, Games etc.