Google wants to know who are using their services. Therefore, before they allow you to download maps from their site, they want you to sign up. You can do this from here:
http://www.google.com/apis/maps/signup.html
As long as you do not use more than 50,000 page views per day, you are free to use it. In this page you can also read the terms and conditions of use.
Upon registration, you will be assigned a key. This key is linked to your domain. For testing this application, enter http://localhost:8080.