An APK file is an application package created for Android, the Linux-based mobile phone platform developed by Google and the Open Handset Alliance. It is generated using Google's Android SDK, a Java-based development kit.
Android packages contain all the necessary files for a single Android program. These include the code (.DEX) files, resource (.ARSC) files, and the AndroidManifest.xml file. When opened on an Android device, the application files are installed automatically. You can install APK files from unknown sources by following these instructions.
NOTE: APK files are saved in a compressed .ZIP format and can be opened by any Zip decompression tool. You can either rename the file extension to ".zip" and open the file, or you can open the file directly through the Zip application's open dialog box.