What is an APK file and is it safe to install?

An APK is the file format Android uses to package apps. It's like an.exe file on Windows. Apps from Google Play Store are APKs. Installing APKs from strangers or untrusted websites is risky, but the APK format itself isn't dangerous.

APK stands for Android Package Kit. It's the format that holds all the code and resources an app needs to run. You install APKs from Google Play Store every time you download an app — you just don't see the filename. You can also install APK files from other sources if you allow it, but that's where the risk comes in.