To do a quick review, App Inventor has three parts:
On the web: App Inventor Designer, the part that we access via our various Google accounts.
On our local computer: App Inventor Blocks Editor (downloaded from Designer by clicking on the Open Blocks Editor button), USB drivers for our devices (how to get and install those coming next), and emulator software (which we just installed as part of the App Inventor software).
A test device: That can either be the emulator on your computer or an Android phone, connected via an USB cable.
Note
In my opinion, the ideal test device is a phone because that's mostly what we'll be developing apps for in this book. In my case, I use my Droid 2.
However, the emulator—software running on our local computer that gives us a virtual Android device to use for testing—is the first we'll look at. In fact, utilizing this virtual device, you can develop apps without even owning a phone.