What is combo as opposed to just doing software update?
The Combo updater includes ALL changes from 10.6.0 -> 10.6.8. The delta updater, it only includes the changes made from 10.6.7 -> 10.6.8.
In theory, both should get you to the same place - 10.6.8. The combo updater is useful if you haven't been updating regularly, or just did a fresh install of snow leopard. That way, you can just install the combo updater - versus installing 10.6.5, then 10.6.6, 10.6.7, 10.6.8 for example.
Some folks think using the combo updater yields better results, even if the delta updater would suffice. I don't have any facts to back this up; I think its based on a couple bad delta updates many, many years ago.