The functionality of mobile devices running the Android operating system, the most used mobile operating system globally, can be extended using "apps" - specialized software designed to offer users the means to use their devices for specific additional purposes. Such apps are compiled in the Android-native APK file format which allows easy redistribution of apps to end-users.
Most apps are distributed through Google's Play Store but many alternative software repositories, or app stores, exist. Alternative app stores use Android devices' "Unknown Sources" option to install APK files directly via the Android Package Manager.
See main article: Google Play. The Google Play Store (originally the Android Market), operated and developed by Google, serves as the official app store for Android, allowing users to download apps developed with the Android software development kit (SDK) and published through Google. The store offers both free and paid apps. Apps exploiting the hardware capabilities of a device can be targeted to users of devices with specific hardware components, such as a motion sensor (for motion-dependent games) or a front-facing camera (for online video calling). The Google Play store had over 50 billion app downloads in 2013 and has reached over 2.96 million apps published in 2020.
Although bundled with most Android devices, the Play Store is only available on devices that are certified within the "Android Compatibility Program". As a result, manufacturers of so-called "custom ROMs", i.e., modified versions of Android, are not allowed to bundle Google apps, including the Play Store, with their software. Compatibility can be restored by installing the Google apps from another source, such as OpenGApps, or using alternative app stores.
In addition to some manufacturers not creating certified compatible versions of Android, some manufacturers have decided to bundle their own app stores, either in addition to the Play Store or as a replacement.
Such app stores include:
App stores that do not rely on pre-installation by the manufacturer are an alternate option for finding Android applications. Apps offered through third-party app stores or websites, created by parties not affiliated with the device or operating system (OS), are also third-party apps.
Such stores include:
This form of the app store is often used by web developers to distribute apps that are not allowed in the Google Play Store; this may be due to an app allowing users wider access to the app system, or offering apps for "niche users" who choose to use only free and open-source software (F-Droid) or prefer to play indie games (Itch.io). Moreover, there are alternative stores that serve to distribute "hacked" versions of paid apps, for no cost.[2]
There are also proxy app stores that act as a client for the Google Play store.