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The idea is that with a very simple syntax, you can do a kind of inline browser sniff for versions of IE and feed them code that other browsers won't see.
The basic code looks like this:
<!--[if IE 6]>
<p>Any HTML you want for IE6 and up goes in here</p>
<![endif]-->
You can use any version number from IE 5 up, including the decimal version number if you need it. Some pretty powerful effects and/or workarounds become possible by bringing in a linked stylesheet just for IE. It can also be handy for dealing with changes in the DOM.
Also there is a related syntax that only REVEALS the code to browsers below a certain version:
<![if!IE 5]>
<p>Please upgrade to Internet Explorer version 5.</p>
<![endif]>
Microsoft Developer Reference [msdn.microsoft.com]