"Patch Tuesday" is the day each month that Microsoft releases their newest security updates, typically the second Tuesday of the month.  This month includes seven updates, including three marked "critical", and four rated as "important".  For a full breakdown, check out the Microsoft Security Bulletin posted on line. 

The critical patches impact Microsoft Windows, Office, and the .NET framework, so this affects just about everyone.  Microsoft is urging Windows users to pay special attention to MS12-034, a "critical" bulletin that patches 10 distinct security holes.  Three of these vulnerabilities have already been publicly disclosed and Microsoft expects to see working exploit code released within 30 days.  The vulnerable code in the MS12-034 bulletin is linked to the Duqu malware that was used to spy on high-profile targets in Iran.

For a quick general description of the updates and their significance, check out the post on ZDNet.

A "critical" patch is defined by Microsoft as "a vulnerability whose exploitation could allow the propagation of an Internet worm without user action".  As a general rule, all critical patches should be installed as soon as possible. Note that many of the patches require a reboot for installation.  Also, if you have a managed service support plan with us, all patches are tested before installation.