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You log into Chrome OS (or 'Chromium OS' to give it its pre-supported name) with just your username, which is your Google Account, and your password. Something stronger may be advisable if this is to be the one barrier between the world and your cloud.
The default startup screen is the Chrome browser, with all the features of the version available on Windows, Mac and Linux. The menu bar has largely gone.
In use, cloud applications work exactly as they do on the desktop, benefiting slightly on smaller screens from the lack of ancillary clutter.