Gmail's new look is based on the "Preview" theme, but it's much more than a theme. There are a lot of changes: Contacts and Tasks are now included in a drop-down menu below the Gmail logo, buttons like Archive or Label are only displayed when you select at least one message, there's a new settings menu that lets you change the display density, gadgets and the chat box are now resizable, there's a new interface for conversations, advanced search options are now displayed when you click the arrow in the search box and you can try HD themes.
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For now, you can switch to the old interface from the settings menu, but Google says that this option will be removed soon.
{ via Gmail's blog }