Fire OS is an unofficial variant of Android that is maintained by Amazon and used in their Kindle Fire series of devices. It is based off of the Android Open Source Project code, and does not contain any of Google's proprietary services. Instead, Amazon created APIs are substituted for Google Play ones, including in-app purchasing, mapping, and push notification services, because they are not open source.
The home screen is organized in a carousel, with the most recently accessed content at the front. Below that is a grid of apps.
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