There are two versions of Internet Explorer in Windows 8.
The desktop version of IE and the Metro version of IE.
The underlying engine is shared.
But their UI is very different.
For those users who will focus on desktop interaction with Windows 8, the desktop version of IE gives them the rich web browsing experience they expect (and have today).
For those users who will focus on Metro (touch) interactions with Windows 8, the metro version of IE gives them a similar, rich experience with an interface tailored for touch users.
Windows 8 can cater to traditional desktop users or touch device users.
Whichever you are, each deserves a full fidelity experience tailored to their interaction modality.
As a result, there are two IE UIs.
The real difference between the two versions, technically, is that add-ons and plug-ins (like Flash and Silverlight) are only supported in the desktop version (just as they are today).
The Metro version of IE only supports Flash, and only insofar as the Flash capabilities integrated in Internet Explorer (which is new).
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