The Android SDK comes with a virtual mobile device that runs on your computer and makes use of its resources. This virtual mobile device is called the emulator. The emulator is basically a configurable mobile device. You can configure its RAM size, screen size, and so on. You can also run more than one emulator. This is most helpful when you want to test your app on different device configurations (such as screen sizes and Android versions) but can't afford to get actual ones.