Google ignored the Metro design language and decided to bring the Chrome OS interface to Windows 8. You'll get an app launcher, a shelf where you can pin your favorite apps, you can add new windows, move them, minimize them, maximize them, resize them and close them.
It's a great way to promote Chromebooks. You run Chrome in an immersive environment, as if it were the only application from your computer.
You can switch between the desktop mode and the Metro mode by clicking the menu and selecting "Relaunch Chrome on the desktop/in Windows 8 mode". Relaunching Chrome on the desktop didn't work well for me: only one of the windows was properly restored. "Chrome remembers the mode you last used. For example, if you exit Chrome from your desktop, you'll automatically be in desktop mode the next time you open Chrome," explains Google.
Chrome doesn't run in Windows 8 RT, but Windows for ARM isn't very popular.