What is the difference between Mac OS X 10.6.5 Update Combo and "not combo"

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).