Google's description is really beautiful: "Chrome Frame renders the Web of the future in the browsers of the past. It's like strapping a rocket engine to a minivan." Google thinks that Internet Explorer is a "browser of the past" that can't handle "the Web of the future", so it needs a plug-in to make it faster and bring it up to date.
Fortunately, Internet Explorer 9 addresses these issues and it will finally make Internet Explorer a modern browser.
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