Of the critical patches, two will plug dangerous security holes in all versions of Internet Explorer since version 6. The third critical update patches a flaw in Windows XP, Vista and Server 2003, another one is solely for Windows XP and then there's also an update for a critical hole in Exchange.
You can find the full details at TechNet. For Windows 8 there are two important updates, one with a moderate rating, one without a severity rating, and two critical update for Internet Explorer 10.