From Google website:
Bring Gmail to your domain.
This special beta test lets you give Gmail, Google's webmail service, to every user at your domain. Gmail for your domain is hosted by Google, so there's no hardware or software for you to install or maintain.
Gmail - 2 gigabytes of storage and search tools that help users find information fast.
Control Panel - Easily manage user accounts, aliases and mailing lists.
Want to be part of this beta test?
Gmail for your domain is currently available as a limited beta. If your organization is interested in helping Google test this service, we'll consider your domain for this beta. You'll need to sign in with a Google Account (or get a new one), and answer a few quick questions about your organization and your email needs.