Android is a free and open source operating system based on linux.  Mainly used in mobile devices, such as smart phones and tablets.  The whole Android operating system is divided into five layers.  They are respectively: kernel layer, which contains some changes customized by Linux Kernel and Android.  Hardware abstraction layer, which defines a set of standard interfaces for hardware manufacturers.  The Runtime and common library layer, which contains the virtual machines and the basic running environment.  Framework layer, which contains a series of important system services.