When developing an Android app, any data file, image or XML file (that is, any Resource or Asset) you use is bundled into your application package (APK) for distribution. The Android Asset Packaging Tool, or aapt, is responsible for creating this bundle, which you can think of as a ZIP file with a particular layout that the Android OS can understand. When your application is installed, whether in development mode or by an end user, this APK file is simply dropped into a special location on the device's (or emulator's) filesystem. 