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Apple describes the update as follows: "This update removes the most common variants of the Flashback malware. If the Flashback malware is found, a dialog will notify you that malware was removed. In some cases, the update may need to restart your computer in order to completely remove the Flashback malware."
Flashback is a malicious program which uses a Java vulnerability to track Mac users. At one point, more than 600,000 Macs were estimated to be infected.
The 1.23MB Flashback removal update can be found here.