Whether Android L has Backward Compatibility? I.e, Whether the Application built in android version <4.4 runs in Android L? Will the Google play store accepts only the application built using Material Design Standard in future?

Yes, Android L can run older apps. Some apps do have some issues, but its not an issue with the OS, but the app. Such apps are very rare (like 1 in 5000) No, material design isn't mandatory. Heck many apps haven't even updated styling to Holo although it came many years ago.

Thanks for A2A. A good programming practice is to support backward compatibility and yes google will do that because of its large number of application base. Coming to the second question about material design then Google may inforce and encourage developers for material design but will not stop uploading without it.

Thx for A2A. Yes, Android L have backward compatibility and can run older version app on it. No, Material design shall not be mandatory in future.

All Android versions are backward compatible. While making new project, you have to select minimum sdk version and target sdk version so it is kind of range of versions that you want to support. Yes, Android L can run older apps because of backward compability. No, anything made using Android L API/material design will not work on anything below L. You will have to check manually in program about the device os and then rewrite the code for each version of android.

They are always backward compatible. It will run even gingerbread apps, if that helps.