Whats the difference between the combo update vs. standard update?

The standard (called a 'delta' update) can be applied to one version back, e.g. the 10.5.7 delta update works only if you are running 10.5.6. The combo update will update from any previous version of the main OS release, e.g. the 10.5.7 combo update works if you are running 10.5.0 through 10.5.6 (but not, for example, 10.4.11).

The combo update can also be reapplied as a troubleshooting step (i.e. if you are having problems with 10.5.7, you can download and run the 10.5.7 combo updater).

If you use Software Update, it will apply the appropriate update (the delta if you are running the immediately previous version, the combo if you are >1 version back).

Ok. 😉

Personally, I always use Software Update, which takes the guesswork out of it...

I use Software Update to notify me of things, but with OS updates always wait for the Combo version and download it directly from Downloads (then after using it gets Archived to an external backup in case it's required again).

Agree. Since the ancient days of 10.1, it has proven to be statistically safer to update with the Combo rather than the Delta. What this means is that, say, 99 out of 100 updates you will not have an issue with Delta updates, but the grief from that single time it fails will more than overcome any savings in time and effort. Plus, when you're trying to recover from a failed update, the first recommendation is to reapply the COMBO update anyway....