Google offers this official Android app development kit for free, it includes a series of drivers, tools and different resources needed to develop apps for the Android mobile operating system, or to run certain desktop apps. Even though this SDK can also be obtained in the Developer Tools pack, where the IDE Eclipse is also included, it can also be downloaded independently.
Android SDK Platform-Tools (ADB) includes several tools to help you take your first steps programing for this platform, including API's brought to you by Google, both for control of the device's features and for integration of all services, apart from a debugger, an emulator for you to test the apps on your PC, and a huge amount of information for you to use when learning to program on Android using Java.
Android is a Linux-based kernel operating system that uses a virtual machine to run the apps, and offers support for all the usual features on modern devices, such as 3G, Wi-Fi, GPS, touch screens, among others which are all now commonplace on any smartphone or tablet.
