There are four builidng blocks for Android Application:

• Activity
• Intent Receiver
• Service
• Content Provider

But Not every application needs have all them.

AndroidManifest.xml is XML file, it contains some information that where you declare the components of your application and what their capabilities and requirements are.

Andriod is built on the open linux kernel. Furthermore, it utilizes a custom virtual machine that has been designed to optimize memory and hardware resource in a mobile environment.