Amazon has always had its own App Store for Android, but to get it, you had to go jump through some hoops, which involved making sure that your device would allow apps outside of the Google Play Store to be installed on your device (which can be done by going to your device's security settings), heading over to Amazon, downloading the app, and installing it, all before you could download apps from the store itself. The reason for this is because the app has been banned from the Google Play Store as it offers an alternative to Google's own app store, so of course Google didn't want to make it easier for users to seek out new alternatives.
However, it has been recently discovered that Amazon has made their App Store obsolete by packaging the functionality of it into their main Amazon app. Six weeks ago, the company released an update to their Amazon app which included the ability to stream Prime Video from your device. Amazon was quite vocal about this but didn't tell us that it had also included an app store in the app itself.
This feature has flown under the radar for those six weeks but has come to light now, as users have begun to discover that there is an option for downloading apps and games from the main Amazon app in the slide-in navigation drawer to the left of the app. However, though this makes it easier for people to have the app store on their devices, users will still have to make sure that their devices are set to install apps outside of the Google Play Store.
What this essentially means is that Amazon sneaked in a complete app store under Google's nose. The main Amazon shopping app has been downloaded 50 - 100 million times, according to the Google Play Store itself, which means that Amazon may just have gained a lot more users than it had before. Now that this information is out, it'll be interesting to see whether or not Google will take steps towards banning this or not.
Download the Amazon app from the Google Play Store now, if you haven't already.