APK stands for Android Package (sometimes Android Package Kit or Android Application Package). It's the file format that Android uses to distribute and install apps. As a result, an APK contains all the elements that an app needs to install correctly on your device.

APK files allow you to install apps on your Android phone. They're similar to the APPX files used to install Store apps on Windows, as well as corresponding package files on other platforms. When you open an APK on your Android device, it contains the instructions to install the app on your phone and provides information about the package itself to your device. Normally, when you visit Google Play to download or update an app, the store automatically installs the APK for you. In this way, the Play Store also acts as a package manager—a tool for easily installing, updating, and removing software on a device. However, due to Android's open nature, Google Play is not the only way to find and install APKs. It's easy to obtain an APK file from elsewhere, move it to your device, and install it manually. See how to sideload apps on Android for a full guide.