Android SDK is a set of tools for creating applications for Android platforms.
It contains everything that the developers need: sample projects with source code, development tools, necessary libraries and so on.
Android SDK also includes a fully functional emulator of an Android platform that enables the developers to test the elaborated application.
The emulator is easy to install and user-friendly. Even non-technical people can try it. But programmers say that it takes some time and effort to install an application on the emulator.