One can think of Android OS as software stack where each layer of the of stack consists of programs that perform specific operating system functions.
The base of the stack is the kernel which house all the hardware drivers. For example, Android kernel includes a camera driver, which allows the user to send commands to the camera hardware.
Libraries and Runtime Environment
The top next layer of software stack includes Android’s libraries. Think of libraries as a set of instructions that tell the device how to handle different kinds of data. For example, the media framework library supports playback and recording of various audio, video and picture formats then a Web browser library and so on.
Located on the same level as the libraries layer, the Android runtime layer includes:
A set of core Java libraries- Android application programmers build their apps using the Java programming language. And, The Dalvik VM which enables every Android application to run in its own process, with its own instance of the Dalvik virtual machine.
The Android OS uses virtual machines to run each application as its own process. That’s important for a few reasons. First, no application is dependent upon another. Second, if an application crashes, it shouldn’t affect any other applications running on the device. This also simplifies memory management.
This layer provides various managers to access all hardware related services. Most of these managers come as bundle of Java classes that manage the phone’s basic functions like resource allocation, telephone applications, switching between processes or programs and keeping track of the phone’s physical location. Application developers have full access to Android’s application framework. One can think of the application framework as a set of basic tools with which a developer can build much more complex tools.
The top of the stack comprises of the applications themselves. This is where you find the basic functions of the device such as making a phone call, accessing the Web browser and accessing your contacts list.