Android application package (APK) is the package file format used by the Android OS for distribution and installation of mobile apps and middleware. You can download APKs from Google Play Store, Amazon App Store, or other third party markets or websites. APKs can be sideloaded (depending on application settings in OS) from signed or unsigned sources. They generally include Manifest information (XML), with compiled sources contained within. They are zip archives essentially which can be expanded.
Directory structure includes META-INF, lib, res, assets, AndroidManifest.xml, classes.dex, and resources.arsc.
Topics should involve sideloading, apk decompiling, changing bundle id, troubleshooting.