Android is an open source mobile operating system designed to run on a variety of hand-held devices, including wireless phones, tablets and netbooks.
Because it is open source, Android is available for anyone to download, change, and use on a variety of mobile technologies. This flexibility of Android allows hardware manufacturers to adjust Android to work to their specifications more easily than other operating systems.
The Open Handset Alliance — a consortium of 47 hardware, software and telecom companies (including Google, Inc.) — introduced Android in November 2007.
Like Apple’s recent mobile offerings, Android comes with its own app marketplace where users can download, often for free, various apps that can then be used on their device. Android is also often tied to a user’s Google account, allowing them to access a number of Google’s own services, including Mail, Calendar, and Voice.
Source: Welcome to Android (PC World)