If you haven't made the upgrade to OS X Lion yet, Apple has a couple updates available for your Snow Leopard-running Mac. These updates come in two flavors, depending on whether you've updated to OS X 10.6.8 already or not.
For those running 10.6.7 or below, there's Mac OS X 10.6.8 v.1.1. This 453 MB update bundles together the already-released Mac OS X 10.6.8 update with a supplemental update, also issued today. If you haven't updated to 10.6.8, that's the update you want.
If you've already updated to 10.6.8, you want the Mac OS X 10.6.8 Supplemental Update, which fixes a couple issues from the 10.6.8 update and also appears to bundle in the Migration Assistant Update that Apple released just before Lion's launch. This update is just over 10 MB.
If you're still confused about which update you want, the easiest route is to just run Software Update from the Apple Menu and let it figure things out for you.
Apple OS X Yosemite