An APK (Android Package) is the file format used to distribute and install apps on Android devices, containing essential resources like compiled code, app resources, and cryptographic signatures. Typically distributed via the Google Play Store, APKs are tailored to specific device configurations to optimize file size and compatibility. However, APKs can also be manually installed through sideloading, requiring users to enable installations from unknown sources or use tools like the Android Device Bridge. While APKs from reputable sources are generally safe, downloading them from unverified third-party sites poses security risks, as they might contain malware. Additionally, APK bundles, which include APK splits, offer a more efficient distribution method by delivering only necessary components based on device configuration but can be challenging to install without third-party assistance. Running APKs on non-Android systems like Windows or iOS is limited, with some support on ChromeOS and deprecated support on Windows through the Windows Subsystem for Android.