1. An AVD is an Android Virtual Device. It represents an Android emulator, which emulates a particular configuration of an actual Android device.

2. The android:versionCode attribute is used to programmatically check if an application can be upgraded. It should contain a running number (an updated application is set to a higher number than the older version). The android:versionName attribute is used mainly for displaying to the user. It is a string, such as “1.0.1.”

3. The strings.xml file is used to store all string constants in your application. This enables you to easily localize your application by simply replacing the strings and then recompiling your application.