Android
Android (https://www.android.com/) is an open source mobile operating system based on a modified version of Linux. It was originally developed by a startup named Android, acquired and championed by Google in 2005.
According to the report by Gartner Inc. (American IT research and advisory company), in 2017 Android and iOS account more than 99% of global smartphone sales, as shown in the following chart:

Smartphone operative system market. Picture created by www.statista.com.
Android in a nutshell
Android is a Linux-based software stack divided into several layers. Those layers, from down to top are the following:
- Linux kernel: This is the foundation of the Android platform. This layer contains all the low-level device drivers for the various hardware components of an Android device.
- Hardware Abstraction Layer (HAL): This layer provides standard interfaces that expose hardware capabilities to the higher-level Java API framework.
- Android Runtime (ART): It provides a runtime environment for...