Some people have had a bad experience in the past with installing Microsoft updates so we recommend that you wait before installing if possible. Although, the security improvements may be worth the risk that your software will stop working.
The security issue is only relevant to 2008 users. It is described as: “Vulnerabilities that an attacker can use to overwrite the contents of your computer’s memory with malicious code.”
According to Microsoft the Office 2011 update will do the following:
It fixes a problem with automatic configuration in Outlook.com
It addresses an issue with importing data from a PST file from an email message
If fixes an issue with distribution lists
Problems with moving messages between folders of Gmail accounts in Mac Outlook are addressed.