What is a domain?
To sign your organization up for a Google enterprise product, you need an Internet domain name. This is the unique name that appears after the @ sign in email addresses, and after www. in web addresses. It typically takes the form of your organization's name and a standard Internet suffix, such as yourbusiness.com or stateuniversity.edu. You can use a domain name that you already own. Or you can puchase a domain when you sign up for Google services.
Using your domains with Google enterprise services:
When you sign up for a Google enterprise product, you provide the primary domain name you want to use with your services. This must either be a domain name you already own, or you can purchase a domain during the sign-up process. Choose this name carefully as it becomes the main brand for your Google services and you can’t change the name later.
This lets users access your Google services or business web sites at easy-to-remember addresses such as mail.yourdomain.com, www.yourdomain.com, or simply yourdomain.com.
If your organization owns multiple Internet domains, you can add these to your Google account at no additional cost. That way, users at all your domains can use your Google services. For example, a retailer might have domains for different product lines, or a state university might own a domain for each campus.