Android is a software stack for mobile 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.

Can you be a bit clearer with your request? There is a ton of information on the net r.e. Android. But I'd start at android dot com if I were you. Here is an article from a few days ago speaking to the competitive nature of the OS battles... WSJ: Android Tops iPhone in Primary Phone Usage | Mobile Marketing Watch

It helps if you know Java already. I would study Java first if you're not familiar with it. Then, just explore the Android API at the official documentation: Package Index | Android Developers

If you're really serious about this, buy a book or take some classes.

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also its a flexible system, i own a Android device , Xperia , always love it