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.