Just wanted to clarify a bit more than what the article mentions. Not only do Android devices allow you to install an app from an .apk you downloaded (or copied to sd card, or emailed as attachment to yourself, or put in your dropbox, or sent over usb through adb, etc etc), there are more app marketplaces than just Google Play.

That article mentions Amazon appstore (free premium app every day anyone?), but there are many others. https://f-droid.org/ is one I always use (it contains free/open-source apps only), and I guess mikandi (an adult app store) is still around.