What is an Android App?

Android App is a software designed to run on an Android device or emulator. The term also refers to an APK file which stands for Android package. This file is a Zip archive containing app code, resources, and meta information.

Android apps can be written in Kotlin, Java, and C++ and are run inside Virtual Machine. The official development environment is Android Studio.
Distribution and Installation

Apps are normally distributed through app markets such as Google Play Store, so it is possible to enable installation from APK file or via USB connection in device settings.

To install or distribute APK in stores, it should have a unique package name (e.g., com.example.app) stored in the meta-information. After installation, this name is registered with Package Manager. For apps published on Google Play, package name can be seen in the URL:

https://play.google.com/store/apps/details?id=com.android.chrome

To install an app from a PC, a command-line tool ADB (Android Device Bridge) is used. The tool is a part of the Android SDK and is invoked by Android Studio when running the app.