Basics about APK Files and What You Can Do with Them
What’s an APK? This question may sound trivial, but it’s actually significant. You may think that an APK is just the format of those fancy files that you can install on your phone. You’d be right, but there is a while lot more that can be done with Android’s application format in addition to (hopefully not blindly) installing them.
Android applications are nothing more than archives with various folders encrypted during the compilation process. With proper tools like the legendary Apktool, you are able to decompile them, make your changes, and compile them back–though this last part doesn’t always work in every case. You can learn more about the APK, its structure, and how to edit them by reading a newbie-friendly guide by XDA Senior Member Aman_Arora. The provided resources can help you understand the Android ecosystem and hopefully allow you to edit your very first closed source application like many here on the XDA forums do.
This guide is a good starting place for beginners. You should know that APK editing requires deep knowledge of Smali assembler language, so you should look for some guides, books, or code references to make your own changes. For now, you can read more about the APK files by heading over to the All you need to learn about an APK forum thread.