Android SDK (Software Development Kit) is a package that includes tools and APIs necessary to develop applications that can run on Android devices (Google operating system for mobile phones).
This SDK includes libraries, debugger, an emulator to test applications, examples, etc. It can be integrated with development environments such as Eclipse (through the ADT plugin) or conNetBeans. You must download Eclipse and install it, prior to installing the Android SDK and the Eclipse plugin. You can check out the following Android tutorial in Spanish.
To simplify the installation of the tools mentioned in the previous paragraph, now Google offers a package called ADT Bundle , responsible for downloading and installing everything the user needs to start developing applications for Android: Eclipse (with the ADT plugin included), Android SDK Tools, Android Platform-tools and the emulator.
The kit also has a complete documentation, very useful for programmers with no experience in this platform, which details several aspects, such as how to implement a graphical interface, database access (SQLite), the Communication with different components (such as the camera, GPS, accelerometer, etc.), and several other aspects. Applications are programmed in Java language and run on a virtual machine called Dalvik.
Android SDK Tools (Revision 24.4.1) is the latest version (updated October 2015), which is available for Windows, Mac and Linux. You can download it for free.
In case you opt to download the entire package of tools, you must choose the second link, and keep in mind that the file weighs about 450 MB. The link takes you to the official page, where you have to accept the conditions and select the type of system (32 - 64 bits) if the version for Windows is intended.
Download (ADT Bundle)
Download (SDK only)