A file containing a ready-to-install Android app. When the APK file is opened on an Android device, the Android OS uses the information inside to install the app, making it ready to use on that device.
An APK file is a way to install apps that are not available through the app store on the device (if it came with one) such as the Google Play Store.
Google attempts to keep malicious software out of its Play Store, providing some level of safety and security. Apps installed manually using an APK file do not have this protection. That is why, by default, phones that ship with Google's Play Store cannot install apps from APKs. But this setting can be changed by the user. Users should be careful to only install APKs downloaded directly from trusted sources, or risk installing a virus or other malicious software.
APK files can be downloaded from web sites using the device's web browser, or they can be transferred to the phone's storage by a local method such as memory card, Bluetooth, or USB cable.