Android is a new Mobile OS (Operating System) developed by the Open Handset Alliance, a group of more than 30 technology and mobile companies. The first complete, open, and free mobile platform. Android has 4 key-features that differentiate it from other mobile OS's:

Open

Android allows you to access core mobile device functionality through standard API calls.

Breaking down boundaries

Combine information from the web with data on the phone -- such as contacts or geographic location -- to create new user experiences.

All applications are equal

Android does not differentiate between the phone's basic and third-party applications -- even the dialer or home screen can be replaced.

Fast & easy development

The SDK contains what you need to build and run Android applications, including a true device emulator and advanced debugging tools.

More information can be found at Google's official Android page.