An APK (Android Package Kit) is the file format used to distribute and install applications on Android devices. It contains all the elements an app needs to run, including code, resources, assets, and certificates. APK files are essentially a type of compressed file, similar to a .zip, and can be installed manually side loaded or downloaded through the Google Play Store. Developers use APKs to test and distribute their apps outside official app stores. While convenient, installing APKs from unknown sources can pose security risks, so it’s important to ensure the source is trustworthy before installation.