APK: Android Application Package
AAB: Android App Bundle

Google has recently announced that developers need to upload AAB instead of APK format on the Play Store. This new update will allow developers to provide highly optimized apps for different mobile users.

AAB (Android App Bundle) basically a bundle of various APKs and other modules that help Google to provide highly optimized APKs according to the user’s android device. As of now, there are millions of android smartphones, every smartphone has different hardware functionalities like differences in process power, screen size, ram, and more. This leads to various errors in different smartphones. Therefore, to counter this type of error, Google has told developers to make a bundle of various APKs according to devices, and then Play Store will decide which APK should be installed in the user’s phone after inspecting its hardware and other basic information.

In the end, you’ll end up installing an APK, but AAB is merely for developers to help Google provide optimized APKs to its users.

Google Play uses the app bundle to generate and optimize APKs for distribution for different device configurations and languages. This makes your app smaller (on average, 15% smaller than a universal APK) and faster to download, which can lead to more installs and fewer uninstalls.