An APK (Android Package Kit) is the file format used to distribute and install applications on Android devices. It contains all the necessary elements for an app, including code, resources, assets, and a manifest file. APKs are similar to .exe files on Windows. Users can download APK files from the Google Play Store or third-party sources. Installing APKs manually is known as sideloading. It's important to only install APKs from trusted sources to avoid security risks or malware.