When new updates are issued for OS X or when people are troubleshooting some aspects of the OS, many times the use of a Combo updater is recommended instead of a standard updater for OS X. If you are unfamiliar with the updating options that Apple provides for OS X, then these recommendations could be a little confusing.
Overall updates for OS X are distributed in two major ways: through Software Update and as Standalone Updaters, with the standalone updaters being divided into "Delta" (or "Standard") updaters and a "Combo" updater.
Apple's Software Update utility is the most common and recommended method for updating your Mac. It is available in the Apple menu and will analyze your system for the files you need in order to apply the latest updates. This is beneficial because it minimizes the download size, consolidates multiple updates into one update routine, and performs the update automatically.
While recommended for everyday updates, Software Update has a few drawbacks. The first is that it only provides you with the latest versions of updates, so if you want to upgrade a fresh OS X 10.6.0 installation to OS X 10.6.5, you cannot do this with Software Update because it will only provide you with version 10.6.7 (the latest version to date). In addition, since Software Update will only give you the files your system needs to be updated, then if you have experienced a problem with an update and need to reapply it, Software Update will read your system as already being updated and will not provide you with any more options.
The standalone updaters (both Delta and Combo) for all of Apple's software updates can be downloaded from the Apple Support Downloads site (search for the version of OS X you want, such as "10.6.6"). Both of these update options have their uses, depending on what needs to be done.
The Delta updaters (delta being the triangular symbol for "change") contain only the files that Apple has changed since the previous version of the software. Therefore, if you have the Delta updater for OS X 10.6.7 you will only be able to apply it on systems that are running OS X 10.6.6 and not on any running OS X 10.6.5 or earlier.
This differs from Software Update, which will tailor its updates for your specific system and update you to the latest version regardless of what your current configuration is. While Software Update may seem more practical in this respect, the Delta updater includes all the files needed to update any Mac from the prior version to the one represented by the updater, and therefore can be used to quickly update multiple Macs instead of having to wait for each to download the same software (though with broadband Internet connections this is less of an issue).
Apple's Combo updater includes all the files necessary to bring a major release of OS X from any previous version to the one represented by the update. For example, if you have the OS X 10.6.7 Combo updater then you can use it to update any Mac from any version of OS X Snow Leopard to OS X 10.6.7.
While Software Update will also provide you with this option, the Combo updater, like the Delta updater, is more universal and can be applied to any Mac system, regardless of its hardware or software configuration. Additionally, it can be reapplied to a Mac that has already been updated. Because of this, a Combo updater is frequently used when troubleshooting odd problems in OS X since it can be used to essentially replace a number of system files with fresh ones, without requiring a full reinstallation of OS X.
If for some reason an update has not worked properly and you are experiencing problems with your system after updating, or if files on the system have become corrupted and you are experiencing odd slowdowns, then applying the update again using the Combo updater may replace the files and fix the problem. It is an easy initial step to do when experiencing some problems before going to the extent of performing a full OS reinstallation or restoring from a backup and running the updaters again.