Because of this exploit you can get multiple crippling viruses on your computer by just visiting a website. This exploit does not require you to download a thing, which is one reason why it is dangerous.

The WMF exploit uses a specific dll in Windows to execute installation commands without your knowledge or consent. This ability was added to Windows to perform specific printer commands after a print job had been sent. This file is now being used to install these deadly viruses.

As always, make sure your virus definitions are updated. Below are links to more information about this vulnerability. No official fix has been released by Microsoft at this time. Their are unofficial fixes out already, but always use caution when using any non official patch or guide.

An official patch is slated for January 10th.

Microsoft Acknowledgement
United States Computer Emergency Readiness Team
WMF Weblog Info
Trustworthy Computing