Android is a mobile operating system currently developed by Google, based on the Linux kernel and designed primarily for touchscreen mobile devices such as smartphones and tablets. Android offers unified approach application development for mobile devices.
Android is an open source operating system. Google has made the code for the low-level "stuff" as well as the needed middleware to power and use an electronic device, and gave Android freely to anyone who wants to write code and build the operating system from it. There is even a full application framework included, so third party apps can be built and installed, then made available for the user to run as they like.
If you have any doubt, let me know...