It's rare for Microsoft to issue an out-of-band security patch (one released on a day other than Patch Tuesday) but the one out today corrects several very serious issues, at least one of which hackers are already exploiting.
What Do These Security Updates Do?
These patches from Microsoft update several individual files critical to the functioning of Internet Explorer.
Do I Need These Security Updates?
Please install the update correct for your version of Windows even if you don't use Internet Explorer.
Most of these issues were given a Critical severity rating, meaning if you don't install the patches, it's possible that someone could, under the right circumstances, access your computer remotely.
More information on these updates are available from Microsoft: