The IE patch fixes potential remote execution vulnerabilities in the browser -- holes that let outsiders take control of your machine if you click the wrong Web link, yet this was one of the patches Microsoft tagged only as "important."
It's more than that -- IE users should browse their way, fast, to the IE patch page.
Once IE's patched, scroll on down to the others (half of which you can ignore if you're not running Vista.)
Microsoft can, of course, label their patches however they want to -- but in my opinion, any browser vulnerability is de facto critical simply because of the ubiquity of browser-use... and mis-use.