Android is an open-source, Linux-based operating system that is typically found on mobile phones and tablets, but is also sometimes found on laptops and other media devices.
Android operating system represents approximately 50% of the cell phone market in the USA, 70% worldwide. It also represents approximately 50% of the tablet market.*
When developing websites or applications for mobile devices (including tablets), the Android operating system is something to take into consideration due to its large market share and the increasing use of mobile devices for accessing networks.
That said, Android is designed to run a very wide variety of devices that vary in CPU speed, available storage and RAM, and differing screen sizes and software configurations. Compared to iOS devices, this can make it more difficult to predict the experience of an Android end-user.