One of the many changes of the new ADT 14 Preview is a new user interface for the SDK Manager.
By default, the SDK Manager now groups packages by Android API, make it easier to figure what you have versus what you can update:
The view by API automatically fetches and mergse add-ons and extras contributed by third party manufacturers. This makes it trivial to see what add-ons you could install for API 8 for example. You can also sort packages by repository, which gives you a view similar to what the previous SDK Manager used.
Under the hood, the new SDK Manager knows how to process some new packages types. For example starting with Tools R14, we'll be able to ship system images separately from platform packages, allowing us to provide smaller and more flexible updates.
The page list on the side is gone. The AVD Manager has been split out into its own window, which you can access using a separate icon in ADT, a separate Explorer icon if you used the installer on Windows, or using the tools menu in the SDK Manager:
Note that the AVD Manager hasn't really changed. It will also receive a UI revamp in a near future.