The security issue patched in the update was a fairly serious one, at least in terms of the damage is could do. A malicious person could create a QuickTime movie with a Quartz Composer object that could both gain access to local data and "send it using an encoded URL to an arbitrary web location."
The update repairs this security hole by modifying the QuickTime Quartz Composer Plugin to prevent access to remote web locations
The update weighs in at 26.6 megabytes, and can be downloaded through Software Update (where it is a 25.7MB download). You can get more information on the QuickTime 7.0.1 update page, and Apple has provided a separate document detailing the security issue fixed in the update.