How to download Android apps in APK file format

As many of you know, installing an Android app is a simple, straightforward process – you open the Play Store, find the software you need, and press that big green "Install" button. Android apps, however, also come in the form of packages which are installed manually, not via Google's app store. These packages have an ".APK" file extension, and their practical uses are numerous. For example, one can have offline backups of apps stored as APKs. Even if the app in question gets pulled from the Play Store (like what happened to Flappy Bird), it can still be installed from an APK file. Also, APKs are ... Drop your comments and additions..Below