IE5 interprets the line:
As a '}', which in CSS defines the end of a ruleset. Since browsers after it interpret it as merely a quoted string containing a }, it's ignored.
Therefore, you can put IE5-specific code before that line, and then you can put the >IE5 code after that line, thus letting you correct for IE5's box model problem.
Tell IE5 that the width is 600px, then tell everything else that the width is 498px.
The last bit, with html>body, tells older opera versions to use 498px too, as it doesn't suffer from the box-model bug, but DOES have the quoted brace bug.
Clear as mud? Heh.
Here's a better explanation: http://tantek.com/CSS/Examples/boxmodelhack.html