I am taking this information from Google’s Android site just for easy browsing and record keeping of myself and other peoples, All information is owned by Google and I mean NO violation of their copyright, While I cannot guarantee the accuracy of information provided here, for more detailed information please visit http://developer.android.com/guide

• Android components are Application, Application framework, libraries, Core libraries and Linux kernal.
• Android was built on linux kernal v2.6 which has four basic functioning security management, process management, network stack and driver module.
• Android application are built on Java language and runs on Dalvik Virtual Machine those files are .dex extension
• Application are packaged as .apk extension Each of them runs as process and creates instance of Davik virtual machine.
• With its multi-user capability Android allows unique users id to assign with each running application that runs on its own container limited by the permission of that user.
• Due to every Android apps are running in separate instance of VM they are isolated and security risk and failure are minimized.
• Application can access resource outside its scope by sharing same user id with other process or requesting to Resource manager library.
• Application components are divide into four types 1 Activity -Front end  functionality it provides using user interaction, 2Services – Backend operations to support activity, 3 Content provide – access data from other applications, 4Broadcast receiver – receive triggers system wide
• Android system are multi entry point
• Android applications are invoked as services or activity by Intent Call from other components/application, some applications for example content provider cannot be started by this method but you will have to go through contentResolver