What is Android?

                   Android is a software stack for mobil devices that includes an operating system, middleware and key applications. The Android SDK provides the tools and APIs necessary to begin developing applications on the Android platform using the Java programming language.

Features

Application framework enabling reuse and replacement of components
Dalvik virtual machine optimized for mobile devices
Integrated browser based on the open source WebKit engine
Optimized graphics powered by a custom 2D graphics library; 3D graphics based on the OpenGL ES 1.0 specification (hardware acceleration optional)
SQLite for structured data storage

Media support for common audio, video, and still image formats (MPEG4, H.264, MP3, AAC, AMR, JPG, PNG, GIF)
GSM Telephony (hardware dependent)
Bluetooth, EDGE, 3G, and WiFi (hardware dependent)
Camera, GPS, compass, and accelerometer (hardware dependent)
Rich development environment including a device emulator, tools for debugging, memory and performance profiling, and a plugin for the Eclipse IDE
Linux Kernel

Android relies on Linux version 2.6 for core system services such as security, memory management, process management, network stack, and driver model. The kernel also acts as an abstraction layer between the hardware and the rest of the software stack.

Contains high-level classes encapsulating the overall Android application model.

An Android application is defined using one or more of Android’s four core application components. Two such application components are defined in this package: Activity and Service. The other two components are from the android.content package: BroadcastReceiver and ContentProvider.

An Activity is an application component that provides a screen with which users can interact in order to do something, such as dial the phone, take a photo, send an email, or view a map. An activity can start other activities, including activities that live in separate applications.

A Service is an application component that can perform long-running operations in the background without a user interface. For example, a service can handle network transactions, play music, or work with a content provider without the user being aware of the work going on.



                   The Fragment class is also an important part of an application’s design—especially when designing for large screen devices, such as tablets. A fragment defines a distinct part of an activity’s behavior, including the associated UI. It has its own lifecycle that is similar to that of the activity and can exist alongside other fragments that are embedded in the activity. While an activity is running, you can add and remove fragments and include each fragment in a back stack that’s managed by the activity—allowing the user to navigate backwards through the fragment states, without leaving the activity.
                                              
This package also defines application utilities, such as dialogs, notifications, and the action bar.

For information about using some the classes in this package, see the following documents: Activities, Services, Fragments, Using the Action Bar, Creating Dialogs, and Notifying the User.