Android is a software stack comprising not only OS but also middleware and key applications. It is an open source operating system, means developers customize the operating system for each mobile.
Android was released by "Android Incorporation", a small company in 2003. In 2005, It was aquired by the Google.Google formed a group named "Open Handset Alliance" to develop open standards. In that group many companies joined like Samsung, Motorola, mobile network companies, semiconductor companies etc.