Windows 7 has a much different look and feel from Windows XP, and this is mainly due to the Windows Aero interface.
Windows Aero was first introduced with Windows Vista. It is the default visual mode in Windows 7, except for Windows 7 Starter Edition, meant for netbooks. Windows Aero is a translucent interface that uses glass-like effects, and shows the images underneath.
Windows Aero was somewhat controversial with Vista, as it needed significant system resources to properly display. Since many Vista computers were upgraded from XP, they tended to be lower-powered machines without the graphics and processor capabilities to take full advantage of it its abilities. Pretty much any computer capable of running Windows 7 can use the Aero look.
Windows 7 added a number of new capabilities to Aero, including Aero Snap, Aero Peek and Aero Shake.
Windows XP, in contrast, does not offer any translucence, and no 3-D look.