APK
Android's installation file — fundamentally a zip archive holding code, resources and a signature.
Change the extension to .zip and it unpacks like any archive. Inside are AndroidManifest.xml with the app's declarations, one or more classes.dex files with compiled code in Dalvik format, resources, and native libraries per processor architecture.
The signature is mandatory and binds the app to the developer's key. Updates are only accepted if signed with the same key, which is the entire basis for an update not being hijackable. Installing an APK outside the store is called sideloading and is permitted on Android, unlike on iOS — one of the most tangible differences between the platforms.