What is difference between 10.5.6 COMBO Update and 10.5.6 Update ? Both say

Question

Both say +"The 10.5.6 Update is recommended for all users running Mac OS X Leopard and includes general operating system fixes that enhance the stability, compatibility and security of your Mac."+
*Mac OS X 10.5.6 Combo Update* (669 MB)
http://support.apple.com/downloads/MacOS_X_10_5_6_ComboUpdate
*Mac OS X 10.5.6 Update* (373 MB)
http://support.apple.com/downloads/MacOS_X_10_5_6Update
I've never had any issue whatsoever before in using Software Update to handle all such OS updates. This is my first time encountering problem: It says "Restart", so I click Restart; and it starts on that purple default OS background, then starts an install process. But that status bar has been seemingly stopped for over 40 minutes after just advancing about 1/8th of an inch....
So I forced power-off, restarted, found these forums, and one of many seemingly contradictory knowledge base documents says if you're having problems with automatic Software Update, then instead download a standalone installer.
Which is why I ask the question of this thread. Normally, when I see "Combo Update", the implication is that there are multiple incremental updates stacked sequentially on top of each other, and the Combo will ensure that all intermediate updates get installed along with the most current update.
But in this case, the information at the links above does not differentiate in any way WHICH of the two someone should use if their current OS version is 10.5.5 .

Answer 1

That was really very helpful. So for me it begs a followup.
I ran the "regular" installer since I had 10.5.5 prior to running the update. Via the automated Software Update feature, the update failed several times. However, when I came to the support forums here and then downloaded and ran the standalone update file (the regular one), the process was simple, and my system updated to 10.5.6.
I have checked my system profiler and it says 10.5.6 ...
But based on things you have said, it seems to me that I would be adding an extra layer of solid foundation for my installed OS X if I were to run the COMBO Updater now -- even though I am at 10.5.6 -- so as to ensure the thorough code set is in place.
Am I reading correctly? Is that what you would recommend? Because I already earlier downloaded the COMBO update installer file, at the same time I also downloaded the smaller standalone updater file... At that time I wasn't certain which I should run. And I'd much rather take a little extra time now to run the COMBO if it would enhance the stability of the OS on my computer.
So I look forward to your reply.
In a separate posting I made tonight, in another thread, I am annoyed that for a major OS update Apple didn't entrust their consumer marketing division to write the instructions for consumers... so that instead of just that regular ol' automated convenience of set-n-forget "Software Update", in this case it actually communicated to the customer: "You might want to update Safari and iTunes using this automated process, but we also are updating the full operating system of OS X. And thus we recommend you download that file separately and run it from your desktop as follows...."
Apple Support has historically never been in sync with the Consumer Sales side of the business -- the group that is Best in Class in knowing how to communicate with mainstream consumers.
Thank you again for your reply. I wish the Support gods would take your posting and make it STICKY so that 1000 new threads don't have to be started. But that is like praying for competence from U.S. Congress.