Gone is the gray, 3D look. The controls on the dialog also have a new appearance, notice that use of color on the close button in the upper right corner. What you can't see from these pictures is the new hot-spot behavior for the controls. While the visual style for Windows XP can be very different than the current versions of Windows, the overall feel for working with the OS will be very familiar. The menus and controls will be in the same place and will work the same way.
Skins usually allow the skin author to completely change the look and feel of the skinned component.
In a skin, the controls can not only have a different look and feel, but can also have different locations or not be present at all. This level of change is not appropriate for the whole operating system, the way it is for components such as the Windows Media Player. Themes determine what visual style Windows XP will use to interact with the user. The actual implementation is an interesting fork in the road for the structure of some of the user interface code in Windows. On one fork lies the way all previous 32-bit versions of Windows have been implemented.